Add find_entities helper method, for easy conversion between name and ID

This is the function that I'm having to define in each piece of code that uses flapjack diner 1.0.

@ali-graham What are your thoughts on including this into diner before 2.0 comes out?  
I prefer an empty array, nicer if people are just chaining `each` on the end and don't care if the operation is applied to anything.