Use an option for the starting codeblock tab

This allows the tab 0 to remain for 1.4.4 (but it is not selected
initially) so that existing themes that use it have a chance to react.
It also allows those themes to set the option to zero on 1.4.5 if they
have not yet been changed.
Merge remote-tracking branch 'origin/master' into 1.4.5