Add theme support for drush en (enable) and drush dis (disable)

[jackaponte]

While drush en and drush dis both work well for modules, neither works for themes.

Testing with Backdrop 1.13.2, Drush 8.2.3 and Backdrop Drush commands 1.x-1.x (most recent commit).

Results of running drush dis basis when the Basis theme is visible as enabled in both Drush and the Backdrop UI:

drush dis basis
The following projects will be disabled: basis.
    Do you want to disable the projects? (y/n): y

	Success: basis are disabled.

However both drush pml and the UI show that the Basis theme is still enabled.

Results of running drush en basis, whether or not Basis is actually disabled:

drush en basis        
The following projects will be enabled: basis.
    Do you want to enable the projects? (y/n): y

	Error basis does not exist in your Backdrop installation.
	Try downloading basis first with the command: drush dl basis

This happens with the Basis and Bartik core themes as well as with a custom theme.

[serundeputy]

You are correct! On the initial pass of en/dis commands no attempt was made to encompass themes and layouts. With the intention of opening a follow up issue for those features. You did that here!

Let's use this issue for en/dis support for themes.

[jackaponte]

You are correct! On the initial pass of en/dis commands no attempt was made to encompass themes and layouts. With the intention of opening a follow up issue for those features. You did that here!

Ah great! :)

Let's use this issue for en/dis support for themes.

Sounds good. I'm going to give this a look and see if it's something I think I can sort out, at least partially; assigning this to myself for that.