Omissible `iojs-`

[anseki]

I propose a feature that let we can specify nvmw use v2.3.2 instead of nvmw use iojs-v2.3.2. Sometimes, I type nvmw use iojs v2.3.2 by mistake. I think that no duplicate version numbers are existing between NodeJS and iojs.

nvmw throw an error when %NODE_TYPE%\%NODE_VERSION% was not found. It should set iojs to %NODE_TYPE% and try find again when %NODE_TYPE% is node.