Include Option for Filename Suffix

[bitstarr]

Currently all subsetted fonts will have a name like originalfilename-subset.woff.
I would love to decide by my own if the -subset suffix is needed and which string it should be.

It seem like it's hard wired at the moment, see https://github.com/filamentgroup/glyphhanger/blob/master/src/GlyphHangerSubset.js#L87
Sadly I'm not that experienced in node.js to provide an accurate pull request.

[zk-phi]

I worked around with this patch: master...zk-phi:master

Note that suffix cannot start with -, since it's treated as an commandline option.

[zk-phi]

I've just created a PR, but you may also use this patch zk-phi/zk-phi.github.io@1f86c92 with patch-package npm package until it's merged.