sort plugins by priority, so they are executed in order

https://github.com/mojombo/jekyll/wiki/Plugins #Flags suggest that the priority flag will be taken into account when loading plugins. Right now they are loaded and then executed in alphabetical order, according to the file system. I dont see them respecting the priority flag at all.

This is an isolated branch for a prior pull request, thanks for looking into it.  Tests are passing.