Add support for subxpath to be extracted from the results of main xpath

Hello, Marko! =)

So, here is my proposal for supporting evaluation of sub-XPaths that are evaluated in the context of each result of the main XPath.

The idea is to support the following workflow:

1) User tests a xpath to select a list of elements (posts, articles, etc)
2) User tests a subxpath to select a given information for each element (say, post titles)
3) User tests another subxpath to select a given information for each element (say, post dates)
4) User tests another subxpath to select a given information for each element (say, post links)
5) ...

The proposal is a bit minimal -- if you like the idea, we can improve the UI later on.
Does this look any good?

Thanks!