`vtex2 modify` command

[JJL772]

modify should be used to modify existing VTF files.

Options for this command should be:

• -r --recursive - Recurse into directories
• -f --format - Format to convert all files to, if they're not already in the format
• -af <flag> --add-flag <flag> - Add the following flag to each file
• -rf <flag> --remove-flag <flag> - Remove the following flag from each file
• -c <level> --compress <level> - Compress the images with DEFLATE
• -sr <name> --strip-resource <name> - Strip resource from the image. If all is specified, strip all non-critical resources.
• -m <n> --mips <n> - Generate n mips on each texture

[JJL772]

Implemented as an extension of vtex2 convert