Make it easier to turn debugging on/off
Rewrite most of s:GetNextCmd(). Fix some '.' bugs.

In particular, be aware that dots in strings don't end commands, and
aware that dots in ".." (e.g. when defining Notation) don't end commands
either.
Don't raise errors until the first .v file is open

This means people who install the plugin in a ~/.vim/ directory they use
with multiple vim versions won't get an error if one vim doesn't support
perl or is missing coq if they only use .v files in the other vim.
Shorten the line separating hypotheses from goals

Shorten it from 70 '='s to only 38. Much more managable.