Pausable: add pause & unpause roles

[karim-en]

Currently, the features can be paused and unpaused by the one-role PauseManager, which makes it impossible to assign just a Pause role without Unpause.
There is a workaround for this by creating an account with limited access to the pause method and assigning the PauseManager role to it. Unfortunately, this complicates the access management, so it would be good if it would be possible to add additional roles like in the Upgradable.