Previously we were only loading "official" checkers that had been added
to syntastic. Now we load any checker that is in the right directory.
This allows anyone to add custom checkers to any filetype that uses
`SyntasticLoadChecker()`
This is needed to handle compound filetypes since we cant imply the
location of the syntax checker file from the filetype.
e.g. we want to load `syntax_checkers/python/pylint.vim`, but the
filetype is `python.django`. Previously this was causing `runtime
syntax_checkers/python.django/pylint.vim` to be executed.
