[WIP] Add globstar support

[oxinabox]

Closes #19

Right now this is attached to the x extended flag on FilenameMatch,
I do not know how to hook it to glob

[coveralls]

Coverage decreased (-13.0%) to 79.63% when pulling 1e4609f on oxinabox:ox/globstar into ed18d97 on vtjnash:master.

[vtjnash]

This can't actually be hooked up to glob, since that actually will need a different (glob) algorithm to implement it (this doesn't close #19) and doesn't need any changes to fnmatch.

Since this doesn't appear to be in posix or gnu (and isn't actually relevant for globstar), I'm inclined to say no here, if that's OK with you.

[oxinabox]

Saying no to this PR, I am down with. I wasn't sure it was right to begin with
and that was kinda why I openned it still WIP.

Saying no to #19 is giving up on one of the most useful features of bash's Glob that I miss all the time

[schneiderfelipe]

@vtjnash @oxinabox Couldn't this be implemented as a separate macro/function or requiring a flag? This way, the defaults would still comply with POSIX. This is really useful.

[vtjnash]

The problem is that this PR doesn't really do anything useful, not that the concept (#19) is not useful. The implementation is required is just very different than this.