Add have_child_tag to match any child or grand child tags.

[rajaravivarma-r]

• The have_child_tag can be used to match any child or grand child tags.
• Does not alter any existing matchers' behaviour.

TODO: Documentation pending.

[rajaravivarma-r]

@kucaahbe This PR addresses this issue #65 and possibly an alternative approach to this issue #76 too.

The idea is to match child elements strictly, and not match against the parent tag, which is the current behaviour. Right now, I've added have_child_tag matcher which will match against any child or grand child.

Later we can add have_direct_child_tag matcher, to match against any direct child as well.

Please let me know if this is something you want to proceed with, I'm happy to polish this and add documentation.