PrimeFactors: add 'factorize'and 'prime factorize' triggers.

Intended to resolve issue #383.

- Add 'factorize' and 'prime factorize' triggers.
- Use 'factorize 128' as a secondary example query.
- Add minimal tests for new triggers.
Merge pull request #386 from mwmiller/factorize

PrimeFactors: add 'factorize'and 'prime factorize' triggers.