Support rg

[albfan]

https://github.com/BurntSushi/ripgrep

It has same output as ag, so it should be easy to support it

[albfan]

We should abstract the ag#provider so the program, ag, rg can be replaced and some concepts like group: --group --heading are used

This way this plugin can use any search program, and then maybe it should be renamed as search.vim