Add: rules/go_unknown_command for misspelled go commands. (#933)

* - Add: rules/go_unknown_command for misspelled go commands.
- Add: tests/test_go_unknown_command which tests match and mismatch case of rules/go_unknown_command.
- Change: Added description of go_unknown_command to README.md.

* Add: test_get_new_command for testing rules.go_unknown_command.test_get_new_command method.

* Change: go_unknown_command.match now uses for_app decorator.

* Add: get_golang_commands which dynamically gets golang possible commands.

* Fix: cache proper function instead of its result.

tests/rules/test_go_unknown_command.py

@@ -0,0 +1,21 @@
+import pytest
+from thefuck.rules.go_unknown_command import match, get_new_command
+from thefuck.types import Command
+
+
+@pytest.fixture
+def build_misspelled_output():
+    return '''go bulid: unknown command
+Run 'go help' for usage.'''
+
+
+def test_match(build_misspelled_output):
+    assert match(Command('go bulid', build_misspelled_output))
+
+
+def test_not_match():
+    assert not match(Command('go run', 'go run: no go files listed'))
+
+
+def test_get_new_command(build_misspelled_output):
+    assert get_new_command(Command('go bulid', build_misspelled_output)) == 'go build'