Adopt grep -P to improve matches

[oblitum]

• Fix support for matching quotes in URLs
• Don't match whitespace before file paths
• grep -P is needed

This solves one of the issues I've commented at:

• https://github.com/tmux-plugins/tmux-copycat/pull/134#issuecomment-447623916

I'm finally getting this all fixed! Locally at least :)

• Associated tmux-open pull-request: https://github.com/tmux-plugins/tmux-open/pull/38

[oblitum]

Dunno why that single test fails on Travis, the test's match with message works on my machine. I've tried these changes on Linux and macOS.