Introduce the Parser::Context class

This class is similar to AST::YCP::Context in that it transfers context
between method calls walking the tree (in this case, Nokogiri's XML
representation). It is needed because we'll need to transfer more
context soon (see the next commits), so a simple symbol, which was used
until now, would not suffice.
Implement the --extract-file option

This option allows to compile an include file embedded in another file.
This is useful for compiling include files which are not standalone.