Load-on-demand with `on` option
On-demand loading with <Plug> mappings (experimental)

e.g. Plug 'xxx', { 'on': '<Plug>xxx' }
Avoid remapping of temporary <Plug> map
Add test runner
Use <silent> map
On-demand loading for imaps of <Plug> mappings
Merge remote-tracking branch 'upstream/master'

Conflicts:
	plug.vim