Create custom field type for module

[ctorgalson]

My reasoning was that, when placing comments on a site,

• We want them to be applicable to any entity type.
• We want to be able to show them in certain view modes, but not others.
• We probably want to hide them in all/most views.
• We want to be able to enable/disable comments for any individual entity type as well.

With all that in mind, I thought a field was the best choice.