add use_regex option to live_grep

[Kartiku]

Would be useful to have this option for live_grep as it is for grep_string

[elianiva]

Hmm, I think the better solution is #670, enabling regex on live_grep was causing some issue with incomplete regex, that's why it got disabled.

[Kartiku]

very nice. Passing cmdline args will be useful to do things like filter grep by file extension. Will close this