Add some more styling options

- Add method for setting the typeface.
- ~~Remove the title's top margin so that users can have less whitespace above the calendar if they want. Instead add a nonzero default separation.~~
- Add option to hide the header row with week abbreviations. In our case we'd like a global header row, not one for each month.
I'm worried about breaking people's existing uses of this widget by suddenly showing the default divider (which I think is what happens on this branch, correct me if I'm wrong).