Add parameter reason for method report_gallery_item

[ueg1990]

Currently the method report_gallery_item is not reporting post even though output returned is True. Two issues that could have caused this:

1. Imgur is currently not allowing reporting of posts via the API
2. Method define in imgurpython/client.py is outdated as the API documentation (https://api.imgur.com/endpoints/gallery#gallery-reporting) has defined a parameter reason.
   I added the 'reason' parameter as part of this pull request. But even with the changes, the reporting of a gallery item does not work.
3. In the API documentation, the parameter 'reason' is defined as optional while in the imugr web app, when you try to report a post, it asks you to select a reason which means it is mandatory. There is discrepancy there too