Add support for "source" command rather than parsing .vimrc by default
(#165).  It's possible sourcing .vimrc could break Vrapper features so I
don't want to parse that file without the user knowing.
Cleanup the fix for #143 and #157.  This is less code and less special
cases, hopefully it doesn't introduce more defects in the process.
Update unstable update site with a few defect fixes.