Hierarchical support for describe/it

[DamienCassou]

Hi,

I think imenu-extra is a great addition to my toolbox as I'm relying a lot on imenu. Still, I think that describe/it blocks deserve better: as these blocks are organized into a hierarchy, I would like to see this hierarchy reflected in imenu.

Here is the current status:

as you can see, imenu doesn't reflect how blocks are included in each other. In particular, the order of blocks in the file are not respected and an it() block is unrelated to its surrounding describe() block.

To compare, here is how it looks using the newest version of markdown-mode:

[redguardtoo]

ad7a704 add imenu-extra-process-item-function (Chen Bin)