Support Linux/Windows in yarn_help rule

See https://www.dwheeler.com/essays/open-files-urls.html
and https://stackoverflow.com/questions/5226958/which-equivalent-function-in-python/15133367#15133367

tests/rules/test_yarn_help.py

@@ -1,6 +1,7 @@
 import pytest
 from thefuck.rules.yarn_help import match, get_new_command
 from tests.utils import Command
+from thefuck.system import open_command
 
 
 stdout_clean = '''
@@ -50,6 +51,6 @@ def test_match(command):
 
 
 @pytest.mark.parametrize('command, new_command', [
-    (Command('yarn help clean', stdout=stdout_clean), 'open https://yarnpkg.com/en/docs/cli/clean')])
+    (Command('yarn help clean', stdout=stdout_clean), open_command('https://yarnpkg.com/en/docs/cli/clean'))])
 def test_get_new_command(command, new_command):
     assert get_new_command(command) == new_command

thefuck/rules/yarn_help.py

@@ -1,5 +1,6 @@
 import re
 from thefuck.utils import for_app
+from thefuck.system import open_command
 
 
 @for_app('yarn', at_least=2)
@@ -8,6 +9,6 @@ def match(command):
 
 
 def get_new_command(command):
-    fix = re.findall(r'Visit ([^ ]*) for documentation about this command.', command.stdout)[0]
+    url = re.findall(r'Visit ([^ ]*) for documentation about this command.', command.stdout)[0]
 
-    return 'open ' + fix
+    return open_command(url)

thefuck/system/unix.py

@@ -3,6 +3,7 @@ import sys
 import tty
 import termios
 import colorama
+from distutils.spawn import find_executable
 from .. import const
 
 init_output = colorama.init
@@ -35,6 +36,13 @@ def get_key():
 
     return ch
 
+
+def open_command(arg):
+    if (find_executable('xdg-open')):
+        return 'xdg-open ' + arg
+    return 'open ' + arg
+
+
 try:
     from pathlib import Path
 except ImportError:

thefuck/system/win32.py

@@ -26,6 +26,11 @@ def get_key():
     encoding = sys.stdout.encoding or os.environ.get('PYTHONIOENCODING', 'utf-8')
     return ch.decode(encoding)
 
+
+def open_command(arg):
+    return 'cmd /c start ' + arg
+
+
 try:
     from pathlib import Path
 except ImportError: