[Leveler] Auto assign/take away roles depending on their level.

[Skyridr]

Would be nice to have a feature where leveler takes into account the current level the user has, and automatically give the user a leveler up role (if he/she is missing it) and or take away a role if he/she is below a certain level (feature request from: https://github.com/fixator10/Fixator10-Cogs/issues/77)