Allow configuring how --ignore should be passed (or if it should be passed at all)

[dakrone]

helm-projectile shouldn't hardcode how --ignore is passed.

See: emacsorphanage/helm-ag#283 for more information. I'd love to be able to customize a var that changes this to --ignore-file for ripgrep (or rather, not pass it at all since ripgrep uses your .gitignore correctly)

[xiongtx]

@bbatsov Do we need to add projectile-ripgrep and corresponding helm-projectile-ripgrep commands, since we have projectile-ag, projectile-ack, etc.? Or try to provide a single command as an interface to many search tools like helm-ag tries to do?

Either way, @dakrone, a patch would be welcome.