Added issues search command:

Optional parameters:
- ``assignee``: Finds issues that are assigned to a certain user
- ``mentions``: Finds issues that mention a certain user
- ``commenter``: Finds issues that a certain user commented on
- ``involves``: Finds issues that were either created by a certain user, assigned to that user, mention that user, or were commented on by that user
- ``labels``: Filters issues based on their labels
- ``state``: Filter issues based on whether they’re open or closed