Support checker name as argument to :SyntasticCheck
Add completion for checker name in :SyntasticCheck command
remove some old (now unused) code from the main plugin
add a class to encapsulate loclists

Add SyntasticLoclist class to wrap up loclists, and move loclist query
methods from syntatic.vim to the new class.

Make SyntasticChecker#getLocList() return a SyntasticLoclist.
Merge remote-tracking branch 'dbarnett/cmd_arg'
make a few refactors

Add CurrentFiletypes function as this logic was getting used in a couple
of places and needed a home/name - mostly because of the bizarre
substitute call that needs explanation.

Move the code to get a syntax checker by name into SyntasticRegistry
dont allow checkers with duplicate names for the same filetype
dont call setloclist() until the user calls :Errors