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

thefuck/rules/yarn_command_not_found.py

@@ -0,0 +1,33 @@
+import re
+from subprocess import Popen, PIPE
+from thefuck.utils import for_app, eager, get_closest
+
+regex = re.compile(r'error Command "(.*)" not found.')
+
+
+@for_app('yarn')
+def match(command):
+    return regex.findall(command.stderr)
+
+
+@eager
+def _get_all_tasks():
+    proc = Popen(['yarn', '--help'], stdout=PIPE)
+    should_yield = False
+    for line in proc.stdout.readlines():
+        line = line.decode().strip()
+
+        if 'Commands:' in line:
+            should_yield = True
+            continue
+
+        if should_yield and '- ' in line:
+            yield line.split(' ')[-1]
+
+
+def get_new_command(command):
+    misspelled_task = regex.findall(command.stderr)[0]
+    tasks = _get_all_tasks()
+    fixed = get_closest(misspelled_task, tasks)
+    return command.script.replace(' {}'.format(misspelled_task),
+                                  ' {}'.format(fixed))