Add helm-projectile-rg to wrap helm-rg

[etu]

Might address #61.

Seems to work fine for me at least :smile:

If you have any comments, let me know and I'll improve it.

[xiongtx]

Thanks!

I've opened bbatsov/projectile#1240 so we'll have a projectile-ripgrep to override.

One unfortunate thing is that the helm-rg interface doesn't match those of helm-projectile-grep and helm-projectile-ag:

helm-rg

helm-projectile-grep

helm-ag

But that's something hopefully @cosmicexplorer can address in helm-rg.

[etu]

@xiongtx Yeah, that's probably the right place to address that. But an initial working version is always nice to have. :+1: