Changes

You can use override to specify material XP rate.

Added to levelling logic: Each material now has an XP multiplier, so you
can make some materials level up faster than others. Each material
defaults to 100% normal experience, but can be changed in the material
override module.

You might ask who would ever use this function. I plan on using it in my
upcoming modpack, so there's one interested party, at least. :/
 (commit: f5dd64d)

Fixed Div/0 error.

...it crept in when I switched from using the Arithmetic mean to the
Geometric mean. Sorry.
 (commit: 8779ebf)

Implemented Boni's Todo list

- Replaced "Multer" with a real word.
- Made a config option (true by default) to only make tool heads count
for XP.
- Fixed some spelling errors.
- Moved the material XP gain to a different part of the function: it
works for weapons now.
 (commit: d68bb9a)