Make --invert-match (-v) behave as in ack/grep.

Previously, --invert-match (or -v) would output all lines, with the
all characters not part of a match highlighted. This behavior is
arguably not very useful, and means that `ag -v --nocolor X` simply
repeats its input, regardless of whether X matches anything or not.

In ack and grep, -v will print all lines which do not contain any match.
Arguably, this behavior is more useful. It also has the nice side effect
that the outputs of `ag X` and `ag -v X` won't overlap.

This commit makes ag use the ack/grep behavior by changing how
invert_matches works.