API support

[dkastl]

To be able to use this on our mobile app we need API support.

Full API support would be best, if this is done automagically, but we needed at least:

• GET the list of attachment categories
• GET the category of an attachment
• SET the category of an attachment

[HugoHasenbein]

I'll have a look at it :-)

[dkastl]

@HugoHasenbein , thank you very much coming here and leaving comments!
I wasn't sure, if it would be better to create a fork, then add some enhancements and submit a pull request. If you prefer to discuss issues in your repository, then I'm happy to move there.

@jkraemer told me, that adding API support isn't difficult.

[sanak]

Well, I will try to support this on #5 PR.

[sanak]

@HugoHasenbein
I created HugoHasenbein#9 on upstream, so please check it when you have a time. 🙇‍♂️