Add DSL support for generating patterns

[bfoz]

square waves, dovetails, etc

[bfoz]

This might be better off in the Polyline DSL. I need to think about that...
'repeat' command is for a pattern along a line. Maybe a 'follow' command for patterns along other shapes. Maybe just a follow command...
'square_wave' can be syntactic sugar

Should take arguments similar to drawing a line segment
The block parameters are the step size, the current point along the line segment, the start point, and the end point
repeat from:[], to:[], do |step, current, to, from|
# Logo commands
# forward, left, turn, etc
end

[bfoz]

I've implemented a basic 'repeat' command that seems to work well enough. The implementation isn't pretty. Look into doing this with Fibers.