added blacklist for digest

[NiteshKaradwall]

I just used this gem and need something blacklist thing. Try to add it. Please accept my pull request and do the need full.

[grepruby]

@NiteshKaradwall @alexspeller @DNNX @aaronjensen @kronn
My project does have lots of files and listing all of them in whitelist doesn't seems good. It would be great if we have something like blacklist as well . This will surely reduced overhead for mentioning lots of files in whitelist.

Thanks!!

[aaronjensen]

Look into using negative lookaheads in regexes, you can probably build a regex that acts as a blacklist.