Make default tab selection configurable

[mojavelinux]

Currently, the first tab in the list is selected by default (unless a tab is selected by some other means, such as a URL fragment or sync group). It should be possible to configure/override which tab is selected by default.

One way to do this is to allow the default tab to be specified by its 1-based index in the list.

[tabs,default=2]
====
Tab A:: A
Tab B:: B
====

Another approach would be to allow the default tab to be specified by its ID:

[tabs#tabs,default=#tabs_tab_a]
====
Tab A:: A
Tab B:: B
====

(This becomes simpler if each tab can be assigned an explicit ID, as proposed in #57).

Yet another approach would be to allow the default tab to be specified by its label:

[tabs,default=Tab A]
====
Tab A:: A
Tab B:: B
====

However, this is tricky for a number of reasons. The first is that the label may include non-text content or formatting that makes matching difficult. A possible approach here would be to compare the AsciiDoc source of both values, perhaps after removing a leading anchor. It's certainly possible, but just gets kind of messy. I'd like to get some feedback about whether anyone would actually want this approach before spending the time to implement it.

[ioannisstavrakakis]

I think approaches 1 and 2 should suffice.

[cunka]

I like the 1st option of the numerical index.

The reason is its clear and obvious to a generic basic user of the language.
Thats not to say that options 2 and 3 are not useful.

The 3rd option could come unstuck when the tab names are a few words long.

Targeting by ID is that next level of being specific and would be something that should be considered when use cases are presented. (e.g. styling to a convention, or a set of branding guidelines)