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
Is it private? I assumed its public cuz its documented I guess.. I linked to that because it has authoritative documentation on input formats. Let me know if I should change this.