[Feature] Add ability to specify /sell and /worth multiplier per world.

Summary of changes:


1) Added new config section called world-price-multipliers
2) Modified Worth.getPrice to apply any world multipliers noted in 1


Tested the following scenarios:

1) Completely missing the world-price-multipliers config section (default 1.0)
2) world-price-multipliers section exists but current world in was missing (default 1.0)
3) /sell and /worth in various worlds with different multipliers.

Sample screenshot: http://i.imgur.com/xAIuMuH.png