Add 2 classes: SyntasticChecker and SyntasticRegistry.
SyntasticChecker represents a checker. It holds funcrefs to the checker
func, the highlight regex func and a new `isAvailable()` func (that
essentially just checks if the checker exe is installed)
SyntasticRegistry is responsible for:
* loading checkers
* storing checkers
* fetching the checkers to use according to availability and the users
settings
Motivation/benefits:
* in the current system only one checker can be loaded per filetype
* syntax checkers cant be "chained" together
* the system is hard to add features to since fundamental concepts like
syntax checkers and location lists arent represented explicitly
Things left to do:
* add a call to g:SyntasticRegistry.CreateAndRegisterChecker() to all
checkers
* add an `isAvailable` function to all checkers
* move all checkers into `syntax_checkers/filetype/checkername.vim` -
g:SyntasticRegistry assumes this layout, and its a good idea anyway
for consistency and it makes it easier for users to add their own
checkers
Things to do after all of the above:
* add a LocationList class and move all the filtering functions onto it
* possibly add an Error class that wraps up each item in a loc list
Random notes:
* with the new system you can select the checkers to use with e.g.
`let g:syntastic_python_checkers=['flake8', 'pylint']`
This will try flake8 first, and if no errors are detected it will move
onto pylint.
