Handle wrong ARCH

[yanxyz]

well, I use a wrong installation command by accident: nvmw install iojs 1.8.1, and get a wrong result:

Start installing iojs/latest (1.8.1) to F:\Nodejs\nvmw\iojs\latest-1.8.1

the right one: nvmw install iojs-v1.8.1 and the right result:

Start installing iojs/v1.8.1 (x64) to F:\Nodejs\nvmw\iojs\v1.8.1

The codes below set ARCH to '1.8.0' in the file nvmw.bat:

if not %ARCH% == %OS_ARCH% (
  set "NODE_HOME=%NODE_HOME%-%ARCH%"
)

I know I make a mistake, however should we handle this case?