feat(cgpt): Add min_size argument to OpenDrive()

This enables the caller to either assert the drive is a given size,
extend existing image files to the given size if mode is O_RDRW, or
create new image files if mode is O_RDRW | O_CREAT.

All calls to OpenDrive have been updated to pass a min_size of 0,
preserving the existing behavior.
feat(cgpt create): Add create and size command line options.

Now it is possible for cgpt create to actually create or extend the
disk image it is operating on. This will help simplify some of the image
building scripts.
Merge pull request #7 from marineam/images

cgpt create