[TW-1312] Negative limit: to behave as "tail" instead of "head"

_Renato Alves on 2014-05-02T10:28:48Z says:_

The current limit: filter accepts negative numbers but never displays any task.

Example:
```
task limit:-2

7 tasks, -2 shown
```

Given that the behavior of `limit:` is similar to the unix `head` command I propose that negative numbers are treated as the unix `tail` command, selecting tasks starting from the end instead of the start.

In practice using a negative `limit:` would be equivalent to inverting the sort definition and transform the number to positive.

*PS1:* The feedback message displayed also seems wrong `7 tasks. -2 shown.`. Looks like another form of TW-908.

*PS2:* This request may conflict with the request from TW-173. It could work if task can see that in one form, only 1 number is being passed (current request), while in the other 2 numbers and an operator (TW-173).

*PS3:* `limit:0` Seems to be ignored. Makes sense, but not sure if this expected.