Whitelisting of extensions for consoles

This small edit allows you to supply a number of accepted file extensions for consoles.

When checking for roms, Ice will validate the file path against the whitelist. If no extensions are entered, or the extensions parameter in consoles.txt is omitted, all roms will be accepted.

Reasons to use this:
* Avoid double entries for games that come in a bin/cue pair
* Avoid double entries for Wii games; For big games, images can be split in a .wbfs file and .wbf1, .wbf2, etcetera. By just adding 'wbfs' to your extensions parameter
* Avoid adding savestates / memory cards / ram files as duplicates of your game

My apologies for the unnecessary edits with line breaks - a result of my using Linux. I'll see about fixing that for future pull requests :)