Add support for LinkedIn comments

### Need
[ozgur/python-linkedin](https://github.com/ozgur/python-linkedin) does not provide any support for LinkedIn comments, e.g. fetching comments of a post, submitting a comment on a post, etc.

### Deliverables
Support for fetching and submitting a Linkedin comment.

### Solution
- [ ] add `get_comments()` method

    ```python
    def get_comments(self, post_id, selectors=None, params=None, headers=None):
        # Return all comments beginning with the provided `since` parameter
    ```
- [ ] add `submit_comment()` method

    ```python    
    def submit_comment(self, post_id, comment=None, title=None, description=None, submitted_url=None, submitted_image_url=None):
        # Create comment dict and submit it to the social platform
    ```

#### Notes
?

####eta: 1p