Select field should allow max_items

[jasonvarga]

The select fieldtype should allow max_items: n, similar to relationship fieldtypes.

Right now you can do multiple: true, but you can't set a limit (except manually using validate: 'max:n')

Rather than replace multiple with max_items, we can probably just allow both.

Like multiple: true could assume max_items: infinity
and setting max_items: 3 would assume multiple: true
and if you do both... good for you for being explicit.

This will help simple v2 suggest fields to be migrated over to v3 select fields.