Enhance reStructuredText imenu list to include section numbers in each entry

[pierre-rouleau]

I'd like to be able to see the section numbers of a large reStructuredText document. With a format like this: 1.2.3.4.5.6.7

where:

• 1 identifies the first level (a chapter),
• 2 identifies the level 2
• 3 identifies the level 3,
• etc...

Ideally this would also show up inside the table of content generated by the rest mode.