Allow equations in place of numbers

[Trimatix]

if message content can be parsed as an equation, evaluate it and check the result against the current number.

Example parsing library: https://github.com/silentmatt/expr-eval

[benslv]

I imagine this would be feature you can toggle on and off with some kind of setting, right? Also worth considering any potential security concerns letting users essentially run an eval() command whenever they want (although I imagine that might be why we use that parsing library instead of handling it ourselves).