Audit fonts for license compliance

[ToxicFrog]

It was pointed out in #25 that Source Code Pro is licensed under the SIL OFL with reserved name "Source", which means we can't call the modified version "Liga Source Code Pro". This is not a problem for people who are downloading Ligaturizer and generating their own fonts, but is a problem if we want to keep checking in the ligaturized fonts.

It is likely that we have other input fonts using SIL OFL that are violated by output-fonts.

Probably the best way to deal with this is to check the license for each included font, then write a proper release script that renames fonts as appropriate. This can be part of some build process improvements I've been wanting to do for a while.

[ToxicFrog]

Quick rundown of font licenses (n.b. links are to Google Fonts repo where possible, even when upstream exists, just because that has a nice consistent layout and minimizes the number of deps):

• Fantasque Sans: SIL OFL, no reserved name listed
• Anonymous Pro: SIL OFL with reserved name "Anonymous Pro"
• Camingo Code: CC BY-ND.
• Cousine: Apache 2.0
• DejaVu Sans Mono: public domain
• Droid Sans Mono: Apache 2.0
• Hack: MIT
• IBM Plex Mono: SIL OFL with reserved name "Plex"
• Inconsolata: SIL OFL, no reserved name listed
• Meslo LG: Apache 2.0
• Oxygen Mono: SIL OFL with reserved name "Oxygen"
• Roboto Mono: Apache 2.0
• SF Mono: unknown, but wikipedia implies a very restrictive custom license
• Source Code Pro: SIL OFL with reserved name "Source"
• Ubuntu Mono: UFL
• Edlo: Based on Aurulent Sans, which uses SIL OFL with no reserved name listed.

Summary of licence requirements WRT naming:

• SIL OFL does not permit us to use the "reserved name" in the modified version at all, so e.g. "Source" cannot appear in the name of the ligaturized version of Source Code Pro. Fonts released under the OFL may decline to list a reserved name, in which case modified versions can use the original name
• UFL allows it if the font has not been "substantially changed", in which case it must be named "[original name] [variant name]", e.g. "Ubuntu Mono Ligaturized". If it has been "substantially changed" the name must be changed entirely, but I don't think adding ligatures counts as "substantial".
• Apache 2 and MIT place no restrictions on naming of derivative works.
• CC BY-ND does not permit derivative works in any form. We may still want to include CC BY-ND fonts as submodules but shouldn't generate ligaturized versions for distribution.

[ToxicFrog]

Names should now be license-clean. Some licenses require that we include the original license text with the fonts themselves if redistributing them, which at the moment we're only doing insofar as the license info is embedded in the fonts themselves, which isn't always the case. The easiest fix for this is just to include the fonts as submodules instead of checking them in directly, which automatically gets us their README, COPYING, etc. Since this is something I want to do anyways and isn't strictly license-related, I've created #43 to track it.