New feature: message filtering.

New options: g:syntastic_quiet_messages, and a per-checker version
of it named g:syntastic_<filetype>_<checker>_quiet_messages.
Option g:syntastic_quiet_warnings is now deprecated.
Option g:syntastic_ignore_files now refers only to files that shouldn't
be checked.
Optimise loclist.filter().
Optimise postprocessing functions.