[cf-forms] Finish implementing a range slider

[KimberlyMunoz]

via @cfarm in https://github.com/cfpb/cf-forms/issues/44

Need to add value labels for the new range input element (see #43).

See Retirement website for an example of value label styles.

[anselmbradford]

Range isn't in, is it? I don't see it in the docs.

[jimmynotjim]

No it was never implemented.