add new pencil styles #141

Hi! My girlfriend is a fan of your app. She is a children's art teacher, and really likes your app because it's simple enough to be used by childrens.

The question is: Is it possible to add new kind of pencils/brushes? Simple ones like diagonal, square, etc.

This is a draw she made with the app:

This is a draw from my brother (he's a software developer like me but draws all the time):

Thanks for your work!