Add transparency to tab loading bar

Add transparency of 0.3 to .strip .tabs .tab .loading so it is easier
to pick out tabs when they do not have focus.

TODO: Possibly move from hardcoded opacity levels to a user-config
file or dialog.

Author: jacobsmith

![breach_tabs_loading](https://cloud.githubusercontent.com/assets/3843162/3547301/4f7d0bd2-0894-11e4-85e8-05183073dfb9.png)