Added -x/--line-regexp (match whole line) option

[okdana]

ag already has the -w option, and this seems a fairly obvious complement to it, so i'm worried that this was requested and rejected at some point in the past. However, i can't find anything in the list of issues to confirm that so... here we go.

Just in case it's not super clear, -x is an option described in the POSIX specification for grep which causes the pattern to match on whole lines only. In other words, grep -x 'foo' has the same effect as grep '^foo$'.

This is an option that i miss a lot when using ag, so i thought i'd try to add it. Sorry if it's shit, this is only like the second time i've really done anything in C. I also added some cram tests for both this option and the existing -w.

Added -x/--line-regexp (match whole line) option
- Added -x functionality (like `grep`) for literal and non-literal matching
- Options -w and -x are mutually exclusive
- Added cram tests for -w and -x options