XP Curve

[s0tho]

Is your feature request related to a problem? Please describe.
We need to have an xp system that takes vocal time and textual production into account.

Describe the solution you'd like
Typical formula would be xp_lvl = 5*(lvl-1)^(2)+50*(lvl-1)+100 but ideally we could change it as well as change the range of xp gained per message or per vocal time. We should be able to choose between registering xp only from text, only from vocal or from both.

We should also take into account import from MEE6. So if someone has 45k message with 1m XP on mee6, we should be able to transfer this notion even if the formula and the range isn't the same.

The bot should also handle role promotion when a user attain a defined threshold

any other idea?

[s0tho]

Linked to #4