Add features: Disable Compression, Disable KeepAlives, Cpus

Hi!

Please, accept this pull request for adding this features:

- disable-compression - for speed up benchmark (no need spend cpu time to decompress)
- disable-keepalive      - for test scenario where client don't support keep-alive
- cpus                          - useful option flag for avoid using GOMAXPROCS.

Thanks,
Artem Andreenko.
You should clone this package to `$GOPATH/src/github.com/rakyll/boom` and add your fork as a remote source.