Add yarn_command_not_found rule

This addresses https://github.com/nvbn/thefuck/pull/607#issuecomment-283945505

The code was adapted from the `grunt_task_not_found` rule

tests/rules/test_yarn_command_not_found.py

@@ -0,0 +1,111 @@
+# -*- encoding: utf-8 -*-
+
+from io import BytesIO
+import pytest
+from tests.utils import Command
+from thefuck.rules.yarn_command_not_found import match, get_new_command
+
+stderr = '''
+error Command "{}" not found.
+'''.format
+
+yarn_help_stdout = b'''
+
+  Usage: yarn [command] [flags]
+
+  Options:
+
+    -h, --help                      output usage information
+    -V, --version                   output the version number
+    --verbose                       output verbose messages on internal operations
+    --offline                       trigger an error if any required dependencies are not available in local cache
+    --prefer-offline                use network only if dependencies are not available in local cache
+    --strict-semver                 
+    --json                          
+    --ignore-scripts                don't run lifecycle scripts
+    --har                           save HAR output of network traffic
+    --ignore-platform               ignore platform checks
+    --ignore-engines                ignore engines check
+    --ignore-optional               ignore optional dependencies
+    --force                         ignore all caches
+    --no-bin-links                  don't generate bin links when setting up packages
+    --flat                          only allow one version of a package
+    --prod, --production [prod]     
+    --no-lockfile                   don't read or generate a lockfile
+    --pure-lockfile                 don't generate a lockfile
+    --frozen-lockfile               don't generate a lockfile and fail if an update is needed
+    --link-duplicates               create hardlinks to the repeated modules in node_modules
+    --global-folder <path>          
+    --modules-folder <path>         rather than installing modules into the node_modules folder relative to the cwd, output them here
+    --cache-folder <path>           specify a custom folder to store the yarn cache
+    --mutex <type>[:specifier]      use a mutex to ensure only one yarn instance is executing
+    --no-emoji                      disable emoji in output
+    --proxy <host>                  
+    --https-proxy <host>            
+    --no-progress                   disable progress bar
+    --network-concurrency <number>  maximum number of concurrent network requests
+
+  Commands:
+
+    - access
+    - add
+    - bin
+    - cache
+    - check
+    - clean
+    - config
+    - generate-lock-entry
+    - global
+    - import
+    - info
+    - init
+    - install
+    - licenses
+    - link
+    - list
+    - login
+    - logout
+    - outdated
+    - owner
+    - pack
+    - publish
+    - remove
+    - run
+    - tag
+    - team
+    - unlink
+    - upgrade
+    - upgrade-interactive
+    - version
+    - versions
+    - why
+
+  Run `yarn help COMMAND` for more information on specific commands.
+  Visit https://yarnpkg.com/en/docs/cli/ to learn more about Yarn.
+'''
+
+
+@pytest.fixture(autouse=True)
+def yarn_help(mocker):
+    patch = mocker.patch('thefuck.rules.yarn_command_not_found.Popen')
+    patch.return_value.stdout = BytesIO(yarn_help_stdout)
+    return patch
+
+
+@pytest.mark.parametrize('command', [
+    Command('yarn whyy webpack', stderr=stderr('whyy'))])
+def test_match(command):
+    assert match(command)
+
+
+@pytest.mark.parametrize('command', [
+    Command('npm nuild', stderr=stderr('nuild')),
+    Command('yarn install')])
+def test_not_match(command):
+    assert not match(command)
+
+
+@pytest.mark.parametrize('command, result', [
+    (Command('yarn whyy webpack', stderr=stderr('whyy')), 'yarn why webpack')])
+def test_get_new_command(command, result):
+    assert get_new_command(command) == result