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.
The advantage to this is that no 3rd party modules are required. People
new to Python probably won't have flake8/pyflakes/pylint installed. This
will get them basic syntax checking (no linting) out of the box.
