Improve `theme install` logic to support force-installing a specific version.

Previous `theme install` logic:

* Updates the theme if there's an update available.
* Installs if the theme isn't installed.
* Otherwise, assume the theme is installed.

New `theme install` logic:

* Don't do anything if theme is already installed and no `--version` specified.
* If `--version` is set and is different than what's installed, force install.
* Install if the theme isn't installed.
A Behat test for force-installing a plugin
Behat test for force-installing a specific version of a theme.