FEATURE REQUEST: Write to osm.pbf

[jjcfrancisco]

It'd be great to be able to write back into osm.pbf - this is mainly to be able edit data, filter data or simply create diffs between two osm.pbf files. Thanks

[larsbeck]

@jjcfrancisco we had the same need and added a osm.pbf encoder in #52.
@paulmach First of all let me say thank you for this great library! It is a pleasure to work with!
I put up a PR (#52) but wasn't able to request reviewers. I am not sure about the policy for contributing to this project, so hopefully you are ok with me putting up that PR. If it is and your time permits, it'd be much appreciated if you could review the PR. Thanks!