Read plugins from ~/.config/git/vogue.plugins
Discover plugins by searching the path
Merge remote-tracking branch 'origin/master' into wip-plugin-list

Conflicts:
	git-vogue.cabal

Also remove MonadBaseControl from tests/ while I'm here.
Clarify comment on discoverPlugins
Discover plugins from both env path and libexec

Build a search path from both the environmental variable and the
configured libexec path.
Clarify comment on plugin discovery
Merge pull request #11 from anchor/wip-plugin-list

Discover plugins to be executed