Add support for feature attributes

[ngm]

Feature attributes allow features to have an associated value that can be parameterized at configuration time. They can be used for features that can't be discretely represented as a selection from alternatives. For example, a feature model might have a MaximumNumberOfUsers feature, which would have an associated integer attribute that could be configured via Confeaturator to some integer value e.g. 10.

I think the most likely required types would be - integer, double, and string.

A string example might be a WelcomeMessage feature, which is parameterized to have a particular message for a particular product.

It can be helpful if as well as a type, the attribute can be given a min, max and default value when applicable.

Attributes were introduced by Czarnecki and associates as a way to represent a choice of a value from a large or infinite domain such as integers or strings. An elegant way to model attributes proposed by Bednasch is to allow a feature to be associated with a type (such as integer or string). A collection of attributes can be modeled as a number of subfeatures, where each is associated with a desired type"
-- Czarnecki, K., Helsen, S., & Eisenecker, U. (2004). Staged configuration using feature models.