ice2slice Does Not Honor --output-dir Command Line Argument

[InsertCreativityHere]

Like all other Slice compilers, ice2slice's help dialogue includes:

"--output-dir DIR         Create files in the directory DIR.\n"

and we go through the trouble of parsing this argument:

opts.addOpt("", "output-dir", IceInternal::Options::NeedArg);
...
string output = opts.optArg("output-dir");

But we never actually use this variable. So passing this argument to ice2slice has no effect.

We should fix ice2slice to honor this variable like the other compilers do.