Add a Datalist input to the list of core inputs

Born from https://github.com/justinfrench/formtastic/issues/1047
Includes the input, documentation for YARD and README, and specs
Also adds some custom_macros which are generic and can be reused
This could be simply "… takes data in the same formats as SelectInput, etc", rather than linking to a specific (private?) method that may get moved around in the future.