new formula: nikto

Let's give it one more try - this is a formula taken from old pull request that was never merged due to inactivity from the author - https://github.com/Homebrew/homebrew/pull/28537

I've fixed small things that were asked in the comments to the original PR 
that's a good point. Looking at the source code (https://github.com/sullo/nikto/blob/master/program/plugins/nikto_core.plugin#L1865) -update writes to the database under prefix/var/lib/nikto/databases.

Possibly we should call update during the install step? However the databases change from time to time so users of nikto should have permission to run "nikto -update" themselves