Stockholm format parser/writer

This is in the new parse format that @ElDeveloper created in #235 so merge his first.
I don't think these will render correctly - see [References description here](https://github.com/numpy/numpy/blob/master/doc/HOWTO_DOCUMENT.rst.txt#sections). You should reference these in the *Notes* section with a sentence like: *The Stockholm format is described in [1]_ and [2]_.*