Add -l for displaying filenames only

With this commit, passing -l to spot will cause it to only list the
names of files containing matches, and not the matching lines
themselves.
Add -- for end of options

In order to allow matching strings that begin with a hyphen, this commit
adds the -- option, so that any arguments after -- are considered part
of the pattern to match.
Merge pull request #16 from kbzod/master

Add -l and -- options