Added at_least_one_of parameter validator.

@oliverbarnes and @Morred, check this out pls, I could be missing something, but I don't think this is possible to implement with the current `optional` and `requires`. 

Should this be refactored into `at_least_n_of` and `at_most_n_of`? Sounds weird for an API, to require 2/3 things.

Fixes #691.