Add Lightgrep block benchmarking code, to be enabled with LGBENCHMARK define. This is probably not sufficiently precise for small blocks, but we don't particularly care about small blocks anyway.
use std::chrono for the timing
Some comments and extra guards pertaining to LGBENCHMARK define.
Merge pull request #64 from jonstewart/master

Add a block profiling mode for Lightgrep searching, enabled with a #define