Selecting which transcoders to use

[felipellrocha]

First of all, great work on this library.

I had a question about transcoder selection. I was able to easily compress my textures into a .basis file, but I was surprised about the fact that when I unpack it, I get a bunch of textures out. This seems to be wasteful to me, so I was wondering if there was any way to select which transcoders to use when unpacking. I know exactly which types of textures I'll need, so I was trying to avoid over using the user's hard drive with texture formats I won't be needing.