added support for custom `find` query objects

Hi! Thanks for the module.

On certain occasions I needed to perform custom queries on fields other than the predefined text search fields. This pull request adds support for the `find` parameter to optionally be a mongoose query object rather than just a string. This way, I can still make use of the pagination feature the plugin provides. I implemented the feature, added the documentation to the README file and also added test cases.

If you accept the pull request, I'd also appreciate if you could update the module in the npm registry so I can stop referencing our fork.

Thanks.