Implemented encoding

[jpg0]

Should encode files if supplied the same object format as decoding provides.

[jimmykane]

Is it possible to also create a PR here https://github.com/jimmykane/fit-parser

It's a little but ahead from the original repo.

[pierremtb]

Thanks for the PR @jpg0

As @jimmykane said, https://github.com/jimmykane/fit-parser might be a better place for new features until we sit down and republish from https://github.com/backfit