add a makeprg builder

The problem
---

Some people want to change the syntax checker args and/or executable.
Currently they have to create their own checker to do this.

Solution
---

Create a standard API for building a makeprg that allows users to set
global variables to override the exe or args.

This API is in use in the coffee and python/flake8 checkers - as
proofs of concept.

So, if the user wanted to change the args that get passed to `flake8`
they can now set `let g:syntastic_python_flake8_args="--foo --bar"` in
their vimrc. Similarly they could set `let
g:syntastic_python_flake8_exe='python foo.py'`