Dice Roll

[Thromax]

Closes #85

Summary

Added !roll command.
Users can now roll a set of numbers by using any of the following syntax:
!roll <number> : Will return same number
!roll d<faces> : Rolls a single -faced dice
!roll <dice>d<faces> : Rolls -faced dice

Checklist

• I have read and agree to follow the contributing guidelines and code of conduct in CONTRIBUTING.md.
• An issue was made and assigned to me by a maintainer before opening this pull request.
• I have formatted the code as detailed in CONTRIBUTING.md.
• I have tested all the code in my pull request, including by running BrainBot as a whole.