Build: add version numbers to docs (fixes #1170)

This changes the `gensite` build step to automatically add the eslint version to each rule documentation where it was introduced first. If a rule was renamed, then version number will be the version when it was renamed and not when to rule was introduced.

I’ve noticed that the performance of `gensite` is worse on Mac OS X compared to linux. I also had to increase the `ulimit` on Mac OS X by running `ulimit -S -n 2048` otherwise I get the exception "Error: spawn EMFILE" (see arturadib/shelljs/issues/158).