Add stroke properties

[rlafuente]

Drawbot has miterLimit, lineJoin, lineCap and lineDash (see docs).

Cairo supports these, it should be straightforward to implement. We never did because Nodebox didn't support them and no one voiced the need for them.

While we're at it, I'm ok with using the drawbot syntax (adapted to the lowercase function names), but to me it seems a lot saner to group all the stroke settings inside stroke:

stroke("#dadada", width=50)
stroke("#d0d0d0", linecap="square")
stroke(miterlimit=30)

[villares]

I like that very much!