Support providers as arguments to providers

[kalifg]

Description

Currently this is invalid:

@foo{1..2}
    parentCategory: <getValue(<current()>)>

The way things are parsed right now you cannot nest the <*( provider calls. This isn't a simple fix as the parser right now is a regular expression and this kind of recursion is not something that it can handle in general.

Proposed solution

Tokenize and parse the data so that provider calls can be substituted recursively.