Add --scaling option to override the scaling algorithm used when generating ratios

[Mataniko]

It's not always desirable to scale with antialiasing, especially when using Glue with pixel art. I added the options to override the algorithm and updated the docs to reflect the change.

If a user doesn't pass the --scaling argument, it will default to what Glue already used.

[coveralls]

Coverage increased (+0.009%) to 92.282% when pulling 8797526290abd8d8141bb39bc6c17d1bbc127b90 on Mataniko:master into 37aecf52db6f5fd52cdb9e288fde80fa45a37c19 on jorgebastida:master.