Allow readme syncing to be disabled

[JDGrimes]

There are cases where one might want the other stuff, but would like to turn of the readme syncing.

I think it would be generally useful to add support in the precommit file for an additional configuration file to be loaded, so that it could be configured/extended without hacking it. I guess that might be for another issue, though.

[westonruter]

Yes, I've been thinking of the same thing, in the context of Travis. There should be a config file which would could be sourceed-in to the .travis.yml which allows you to define a branch or commit hash from WordPress Coding Standards that you'd like to validate against.

[westonruter]

In #127 the readme sync can be disabled by adding the config SYNC_README_MD=0