Add --clean to avoid loading ftplugins
Add --clean to avoid loading ftplugins

This uses vim's '--clean' switch to avoid loading ftplugins, since this
can affect behavior during vroom tests. This switch seems to avoid the
issue in my local testing, whereas other things ('-i NONE',
'--noplugin', etc.) don't seem to have the same effect. This falls back
to trying without the '--clean' switch if vim fails to start up.

I can't get vroom running in '--neovim' mode (nvim 0.4.4), so this
doesn't attempt to address the issue in the neovim code path. neovim
appears to define '--clean' differently from vim, so it's possible
neovim may require a different fix.

I verified this still respects vroom's '--vimrc' switch while having the
desired behavior of avoiding plugins.

Fixes #105

malcolmr/vroom,google/vroom