Adding support for discrete values

[noelrappin]

The test video does something like

Put.desc(case user.level
when :staff then 1
when :manager then 2
when :director then 3
when :vp then 4
when :c_suite then 5
end)

Would you be up for a patch that made this something like:

Put.desc(user.level, enum: [:staff, :manager, :director, :vp, :c_suite])

or possibly

Put.desc_by_value(user.level, [:staff, :manager, :director, :vp, :c_suite])

Or whatever you think the API should be.

[searls]

Put.enum(value, order: [:staff, :manager, :director, :vp, :c_suite], nils_first: false)

This seems good