Allow more license Types

[dimensionscape]

Added support for the Mozilla Public License and added an option for other licenses.

[joshtynjala]

Related: #601

I recently ported something from Python using the PSF license (which has an SPDX identifier), and similarly chose Public to make Haxelib happy.

[mikaib]

Heya!

Logo proposed the following:

• Use SPDX license-list-XML with macros to fetch up-to-date license information.
• Deprecate outdated shortnames. (possibly show a warning when haxelib dev is used?``)
• Leverage metadata from the XML to only allow OSI approved licenses.

This should address:

• Libraries using licenses that don’t accurately reflect their actual licensing information.
• The ability to specify the exact license version.

Thoughts?

Edit: seems to be related to #601