Add migrator for bash related names

It migrates
- BashComplete -> ShellComplete
- EnableBashCompletion -> EnableShellCompletion
- BashCompletionFlag -> GenerateCompletionFlag

cli-v1-to-v2

@@ -298,6 +298,23 @@ def _app_init(source):
         'cli\\.NewApp\\(\\)', '(&cli.App{})', source, flags=re.UNICODE
     )
 
+@_migrator
+def _bash_complete(source):
+    return re.sub(
+        'BashComplete:', 'ShellComplete:',
+        re.sub('\\.BashComplete', '.ShellComplete', source, flags=re.UNICODE))
+
+@_migrator
+def _enable_bash_completion(source):
+    return re.sub(
+        '\\.EnableBashCompletion', '.EnableShellCompletion', source, flags=re.UNICODE
+    )
+
+@_migrator
+def _bash_completion_flag(source):
+    return re.sub(
+        'cli\\.BashCompletionFlag', 'cli.GenerateCompletionFlag', source, flags=re.UNICODE
+    )
 
 def test_migrators():
     import difflib