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.
Javascript and json have multiple syntax checkers that can be loaded.
Previously the logic to determine which checker to load was basically
copied and pasted in both. The `go` checker will soon have more
than one option too so remove the duplication by sticking the
common code in the core.
