generate pointer functions for required types

[jbowes]

For types used in options, and eventually in PATCH calls (#17), oag should generated functions that return a pointer of the supplied value, if required.

For example, if an options struct has a *string type as a field, oag should generate a function like:

// String returns a pointer to the supplied string value
func String(v string) *string { return &v}

This way, users of the generated API can easily get pointers to literal values.