Initial commit.

Signed-off-by: David Celis <me@davidcel.is>
Initialize a list of recommendables for a class

If a class includes Recommendable and then calls `recommended` with one
or more things, add them to a class_attribute called `_recommendables`
to being keeping track of relationships similarly to how Rails does it.

Signed-off-by: David Celis <me@davidcel.is>