Scenario:
- a C file is recognized as having filetype cpp
- run :SyntasticInfo
- run :setf c
- run :SyntasticInfo again
The only checker recognized will be oclint. The reason is, oclint is
registered as a c checker when called from cpp initialization, and that
prevents registering other c checkers when calling ':setf c'.
Solution: always initialize all checkers for a filetype.
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.
