Make properties/values prettier in regular index

[astearns]

Right now all of the values but none of the properties are being added to the regular index, which isn't that useful. Since bikeshed should know which values belong to which property, the property could be added to the index with each of its values as a sub-item. So instead of this (from flexbox)

<number> (value), 7.3.1
<number> (value), 7.3.2

you'd get

flex-shrink, 7.3.1
   <number> (value), 7.3.1
flex-basis, 7.3.2
   <number> (value), 7.3.2

[tabatkins]

First half of this work is done now. Multiple index entries with the same text get collapsed into one entry with sub-entries, like:

<number>
  value for flex-shrink, 7.3.1
  value for flex-basis, 7.3.2

Doing it the other way around, where the index entry for 'flex-shrink' includes sub-entries for all of its values, is a bit more difficult, but doable. I'll get to that soonish.

[tabatkins]

Ugh, this'll actually be much harder than I thought. Problem right now is that the for='' values aren't intrinsically connected to other definitions. They're effectively just a disambiguator, with no verification that they actually mean anything right now.

I need to fix that first before I can take this issue any further.

[tabatkins]

Done. It's currently limited to properties, but the index entry will now display entries for the values of the property as well.