Properties for mixin

[akfish]

Codo doesn't support properties in mixins, so there is no way to document code like this:

###
The mixin
@mixin
###
TheMixin = 
  # @property [string] whatever property <- not documented
  prop: 'whatever'

###
The class
@include TheMixin
###
class TheClass

It would be great if @Property can be used in mixins.

[inossidabile]

I like that but the implementation can take time that I don't have atm. PRs are welcome.