Scriptable `select` property; require `return`.

This commit enables script directives in `validate_select`, and allows them to
pass through validation and normalization. It also adjusts `util.exec` so that a
`return` keyword is no longer automatically prepended to the specified script;
the `return` keyword must now be manually specified inside of the JSON form.
Merge branch 'master' of https://github.com/medic/medic-forms

Conflicts:
	tests/fixtures/input/field-properties.json