Use the perl in PATH rather than hardcoding /usr/bin/perl

Developers may use perlbrew or other alternate perl installs in
preference to the system perl.  Hardcoding /usr/bin/perl manifested
problems after I successfully installed all the dependencies but still
ran into missing module errors when running ./tools/run-test.
Calculate line numbers for the matched text

Note that the line numbers are not for the returned snippet of text, but
just the actual matching text.  This is more useful for accurate links
and the snippet's line boundaries may be calculated relative to the
matches line boundaries given the match's internal position within the
snippet.  Consider the "line" values to be the newline equivalent of the
"match" values.
Link result excerpts to the matching source lines on MetaCPAN
Refactor SLAB_SEPERATOR into a common module in prep for use elsewhere
Find the inner file offset via the slab seperator when calculating line numbers

Although un-benchmarked, this is bound to be quicker than querying and
blocking on Redis and no less accurate.
Merge pull request #28 from tsibley/link-excerpts-to-source

Link excerpts to source