[Request] Use exp instead of levels

[Shahelion]

For example, leveling up a single skill is vastly more expensive if I have 60 levels than just a few. Using experience points instead of levels would alleviate this, like how Everlasting Abilities handles costs.

[wise-io]

I second this. It's really my only "issue" with the mod so far.

Just started using Skillable and I'm in love. Love how easy it is to integrate it into other mods! Keep up the great work Vazkii!