Add an `ALECompletePost` event along with everything needed to make it
useful for its primary purpose: fixing code after inserting completions.
* `ALEFix` can now be called with a bang (`!`) to suppress errors.
* A new `ALELintStop` command lets you stop linting, and start it later.
ALE was incorrectly detecting completion results from servers such as
rust-analyzer as wanting to add import lines when additionalTextEdits
was present, but empty.
Now ALE only filters out completion results if the autoimport setting is
off, and one of the additionalTextEdits starts on some line other than
the current line. If any additionalTextEdits happen to be identical to
the change from completion anyway, ALE will skip them.
