Make python checker exe path configurable

I've broken out the command name to a global variable in the python / pep8 / pylint syntax checkers.

This allows me to override which version of these commands i want syntastic to use, without having to adjust my system-wide defaults.

For example, on a Redhat 5 host where invoking "python" actually runs python2.6, i do not want to use the OS facility to override which python interpreter is invoked for the bare "python" command. This is since doing so would break various OS built-in commands which depend on "python" invoking the default python2.6 shipped with the OS.

Hope this helps,

Craig