Allow jiffy encode maps with integer keys?

[gasparch]

Would you consider as a good feature to have option to allow encoding Erlang maps where keys are integers? That would allow skip conversion step from integer to binary which for a big map is a huge overhead (basically recreating map from scratch, which will be used just one time and then thrown away).

[gasparch]

Another question - if it also makes sense to have option to skip certain keys from a map while encoding. Again - to avoid rebuilding whole map just to remove one key on serialization and then throw new map away.

[gasparch]

ping @davisp :)

[gasparch]

submitted PR #150