Random template 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.)

Extra warning: I don't know if I've ever tried using random functions within WASI.

• rand_alpha Random from set [a-zA-Z]
• rand_numeric Random from set [0-9]
• rand_alphanum Random from set [a-zA-Z0-9]

[karthik2804]

Closed by #133