userChrome/tabmenu: Add option to give more place to tabs

This adds and documents an option to make it possible to see multiple tabs next to each other.

Fix #42 (closed) by adding an option that shows multiple tabs (based on previous work by gnumdk) with mcf.multipletabs.

Fix #34 (closed) by adding an option to add a close button to every tab with mcf.multipletabs.showclose.