numeric list functions

[technosophos]

Per #25, implement the Sprig functions for Bartholomew. Rather than file an issue for each function, I figured I'd break these down into small batches. (Note: I am not entirely sure all of the functions can be done with the same syntax. I am no pro in writing the Handlebars functions.)

• until COUNT generate a list of integers from 0 to COUNT.
• until_step COUNT STEP generate a list of integers from 0 to COUNT incrementing by STEP
• seq described here OPTIONAL... not sure we need it. Or maybe we need this and not until...

[karthik2804]

Closed with #133