Export as Homebrew Hub entry

[avivace]

I have a quick and dirty implementation here but I'd like this feature to be done correctly and merged upstream (or at least plugg-able).

This includes two main tasks:

• Exporting the metadata to the Homebrew Hub Game format. The JSON schema is here: https://github.com/gbdev/database/blob/master/game-schema-d4.json
• Export assets (normally a ROM and screenshots) and correctly reference them in the metadata file

Some questions:

• Should we export the web.zip as well for archival purposes?
• What if there are more files?
• Depending on the mapping to the json schema, some question may arise and we could implement changes to the schema to accomodate them (e.g. valid tags)

I'm not sure I'll have the time personally work on this, but I'd gladly put a bounty of $200 (which could be release gradually as we merge features) from the gbdev Open Collective.