Postprocessing functions.

This patch adds an option 'postprocess' to SyntasticMake().  The value
of this option is a list of names.  Each name is translated to a
function syntastic#postprocess#name().  These functions are applied in
order to the list of errors just before SyntasticMake() returns.  They
take a single parameter, the list of errors, and are supposed to
returned the processed list.
Bug fix.
Minor cleanup.