Add :error-filter checker property

Allows syntax checkers to define post-processing functions to modify or
filter errors after parsing.
Collapse whitespace in Emacs Lisp byte compiler messages
Do not implicitly trim error messages
Replace empty error messages with nil
Dedent GHC error messages
Fold errors from included files into the include error [#367]
Merge pull request #397 from flycheck/error-filters

Add :error-filter checker property to apply custom functions to errors after parsing