Improve roll command

[indiv0]

• when no initial number is specified (e.g. d12), assume it's a 1
• add a cheeky message when a user attempts to roll a d0 or a d1
• add special messages for a crit fail or crit success
• add multi-die capability
• ensure the resulting message isn't too long (possibly limit the number of dice?)
• write tests to check that the handler works properly.
• add a notice regarding the minimum and maximum X & Y values to the error message and the help output
• remove hard-coded 4294967295 check with a call to u32::max_value()
• check for numbers larger than 4294967295 in addition to the value itself
• handle very large numbers via the num crate