Added mute and unmute functionality

Automatically following hundreds of people leads to a lot of unwanted tweets appearing on your timeline. You can automatically mute everyone to remove this problem without unfollowing anyone. 

This code provides the option to mute everyone you follow, unless you specify user ID's that will not be muted. 

Incase you change your mind, this code also provides the option to unmute everyone you have muted, unless you specify user ID's that will remain muted.
Updated to show additional functionality

Explains how to use the new unmute and mute functions. Informs that the user must manually edit the code to add 'special users'
Update README.md
Merge pull request #13 from umpox/master

Additional Functionality: Mute and Unmute