Flag/API methods for excluding directories and file reading options

[joshkadis]

--excludes and HugoLunr.setExcludes() take an array of glob patterns to ignore when calling glob.sync()

--fileopts and HugoLunr.setFileOpts() give you some options when reading content files for the index.

[clburlison]

@joshkadis it looks like this repo hasn't been updated in a while you might have better luck getting this merged under @gwleclerc 's fork.