[FEATURE] Attribute generator added.

Now it is possible to scaffold coersion attribute:

  $ hexx attribute some_name
  # This will add Attrbutes::SomeName class with its spec.

It's also possible to set coersion in a model scaffolder.

  $ hexx model cat -a name:string:coerced{cat_name}
  # This will add Attributes::CatName with its spec.

== Addons

* Generators::Attribute.
* `hexx attribute NAME` CLI command.

== Changes

* Model templates (coercions added when necessary to both the model and its spec).
* Generators::Model::Attribute - #coerced attribute added.
* Generators::Model - creates coercions when necessary (using Generators::Attribute scaffolder).
* `hexx model` parameters (`coerced{name}` key allowed for attributes).

== Version

Release 0.1.0 launched with a new generator.