Include Unicode version

[groenroos]

I realise this is not directly available on either data source (1, 2) this script is based on, but it would be handy to include in the JSON which version of Unicode introduced which emoji. This way, emojis that are too recent to be reliably reproduced cross-platform could be easily filtered out from end user applications, if desired.

I guess the script could do multiple passes of the different versions of emoji-test.txt (12.0, 11.0, 5.0, 4.0, etc), and compare contents of each to figure out the introduction version for each emoji.

[amio]

Not sure if this approach is reliable, but let's give it a try :D