Feature: Option to set the initial header level for reStructuredText

[gchriz]

Hi!
I'm using reStructuredText documents more often than Markdown.
rst is more strict with header levels (H1..H6) than mkd and begins with H1 per default but can be configured differently too.
In wok I missed the possibility to set that initial level and so I implemented it here:
gchriz@cb47b5d
It's possible to change that value in the respective wok config file and on a per page basis within the page's meta data. Of course it's documented in the included docs.