[MsgTracker] add autorole support

[sravan1946]

Feature Request

What type of feature are you requesting:

• Cog
• [x ] Command

Describe the feature:

Add a command like [p]mtset admin msgrole 100 @100-msg-role which should assign the role to the user upon reaching the 100 messages in the server

Say what cog it's from - MsgTracker
Describe what the command does - Allow auto assigning of roles on reaching a set number of messages.