Provide wildcard prop matcher

As discussed in #13

Add a property exposed in the module export which is treated as a wildcard for prop matching purposes.

This would allow something like: `subTree('*', { id: 'abc', children: SkinDeep.any }` - which does a complete prop match, but doesn't care about the value of the `children` prop.

Locate the module export and the existing prop-matching implementation, starting with how complete prop matches handle the children prop. Use the example in the issue as the acceptance case, and confirm that wildcard matching ignores only the selected prop while the other props still require an exact match.

Written by the indexing model from the issue text.