Support setting alias attribute on a per-file basis

[Makman2]

As noted/requested on #15, it is not possible yet to make use of the alias feature for resources.

Since this must be settable on a per-file basis, akin to current syntax with flags like COMPRESS etc. inserting custom attributes should be possible. Something like this:

# Set the resources to bundle.
set(RESOURCE_LIST
    info.txt
    COMPRESS ALIAS img1 img/image1.jpg
    ALIAS img2 COMPRESS img/image2.jpg
    STRIPBLANKS data.xml)

The order shouldn't matter. For better clarity, maybe the syntax should be ALIAS=img1, but I am not sure how CMake-ish this would be.