Extend OboOntology to include the Document header tags

[kMutagene]

An OBO document is structured as follows:

<header>

<stanza>
<stanza>
...

We currently only parse [Term] and [Typedef] stanzas, ignoring the document header.

The document header section is consisting of defined key-value pairs, just as the stanzas, see https://owlcollab.github.io/oboformat/doc/GO.format.obo-1_4.html#S.2.1

[kMutagene]

can this be closed @omaus ?

[omaus]

Yep! Done with #35.