Asynchronous grep and integration with the ripgrep/ag/ack search tools

[Yegappan Lakshmanan]

Hi all,

I have updated the grep.vim plugin to use the new asynchronous
features in Vim 8.0. Now the grep commands are run in the
background and the results are added to the quickfix list.
I have also updated the plugin to integrate ripgrep, ack and ag
search tools.

You can download the plugin from:

https://github.com/yegappan/grep

Let me know if you any see issues with these new features.

Thanks,
Yegappan