Added some c snippets and snippet switching.

When trying to write snippets I've found the desire to quickly switch between the python and the generated snippet file.  This is a little fswitch rule that I've used locally for a while and thought it could help others.  I also added a few C snippets that I felt were missing.

For the fswitch rule I expanded SetFswitchVars to allow an optional argument for b:fswitchfnames.  There may be a better way to do this but it's what I figured out with my limited exposure to vimscript!

Let me know what you think!