Add title and title save image codec

[mborgerson]

Distinct from #4, title images and title save images are stored in the XBE in $$XTIMAGE and $$XSIMAGE sections respectively as "XPR0" format images--which are simple 256x256 128x128/64x64 DXT1 encoded images with a header. This format is documented here. For convenience, it would be helpful to have a way to be able to encode/decode these images to conventional formats (ideally without bringing in any graphics library dependencies).

Update: I have this 1/2 implemented locally.

[mborgerson]

Extracting title and title save XPR images as plain .bmps is now supported as of d28d534. See README for CLI usage info.

Since .xbx image files (used for game saves) are also in XPR format, a convenience option was added to convert .xbx image files to .bmp as well.

Converting images to XPR and injecting into XBE file will come later, in coordination with #1.