Add support for buffer-local b:syntastic_checkers var.

This allows more fine-grained control of syntastic checker selection by defining autocmds and a per-buffer `b:syntastic_checkers` setting, as described in #383.

You can use it in an autocmd like this:
```vimL
autocmd FileType python if stridx(expand('%:p'),
    \ $HOME . '/foo/bar/') == 0 |
    \ let b:syntastic_checkers = ['pylint'] | endif
```

Note this allows overriding the checker, not the mode exactly, but you can also use it to selectively disable syntastic with `let b:syntastic_checkers = []`.