Add filtering support in Query

This adds basic infrastructure for filtering (fully functional except,
as usual, for runtime data), and then uses it for node queries.

Since the filtering exports regex matching as an external
functionality, we have to use a regex library. There are many flavours
of these in Haskell (see
http://www.haskell.org/haskellwiki/Regular_expressions

), but since we
want to be as compatible as we can with Python's, we use the
regex-pcre one, which is a wrapper to the widely used 'pcre' library.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Agata Murawska <agatamurawska@google.com>