Implement ycm_quiet_messages options (See #2021)
# PR Prelude
- [x] I have read and understood YCM's [CONTRIBUTING][cont] document.
- [x] I have read and understood YCM's [CODE_OF_CONDUCT][code] document.
- [x] I have included tests for the changes in my PR. If not, I have included a
rationale for why I haven't.
- [x] **I understand my PR may be closed if it becomes obvious I didn't
actually perform all of these steps.**
# Why this change is necessary and useful
See issue #2021. This is a partial implementation based on the syntastic option referenced, supporting the `!` flag and filters of type `regex` and `level`. Also supports filetype specific filters using `ycm_<ft>_quiet_messages`, but I couldn't think of a great way to fall back to syntastic configs for this one.
In terms of usefulness: I've been playing with C# recently, which has a bunch of style warnings that I don't want to follow, and prefer to only have the gutter showing if there are actually errors, or warnings that I *do* want to follow.
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