Feature: Group and limit items in agenda

[Muchenxuan Tong <demon386@gmail.com>]

Hi everyone.

* Motivation
- Sacha Chua proposed a solution for viewing top three tasks by
context (http://sachachua.com/blog/2012/12/emacs-org-display-subset-tasks-context/)
- In the Next view in Things (http://culturedcode.com/), one is able
to setting to see only a limited numbers of top tasks of each project.

I find that it helps a lot for reducing stress by grouping and
limiting the number of tasks you see when reviewing. So I wrote some
code to get something similar to the Next view in Things. The concept
of project in Things is substituted with category in Emacs. See a
snapshot: http://d.pr/i/GXnx

For me this is enough. However, I wonder whether this could be useful
for others. The proposed feature is a general "group by and limit":

- In agenda view, you are able to group todo or agendas with a certain
criterion (category, tags..)
- You are able to restrict the items shown in this group. Optionally,
some special todo states could be unlimited. For example, I only want
to limit the number of TODO tasks, while seeing all the DONE tasks.
For others, maybe they want to see all the NEXT tasks, but limiting
the number of TODO tasks.

I wonder how many people would be interested in this feature.
Otherwise it would just be a waste of time to generalize my personal
taste.