Support comment and/or content shares and visibility other than anyone

Previous submit_share required comment and content fields. The [Share API](https://developers.linkedin.com/documents/share-api) will support comments and/or content so this pull request will only send the values if passed to the method.

Examples:
```
submit_share(comment='Comment only')
submit_share(title='Content only', submit_url='http://example.com')
submit_share(comment='Comment and content', title='Content only', submit_url='http://example.com')
submit_share(comment='Only visible to connections', visibility_code='connections-only')
```

ref ozgur/python-linkedin#16