Text object for comments

[disconsis]

vim-commentary provides a text object gc which can be used to address all adjacent comments.
It would be quite useful if evil-commentary could also provide this.

One very common use case is dgc for deleting all comments. Currently, you would need to visually select all commented lines and then delete (unless I'm missing something).

This is related to #20 but more general, as gcgc is only a special case of using the gc text object.