Add trim command

[foresterre]

• Should be able to trim a colour range, a single rgba colour, or transparency from the edges of an image

For example, if we have an image:

We would like to trim the, in this case transparent, edges from the image s it becomes (approx.):

To clarify, the first image has approx. the cyan bounding box, while the second has approx. the yellow bounding box:

Tasks:

• investigate appropriate inputs, output
• investigate syntax
• implement image engine trim command
• implement cli ops parser
• implement script parser

Help:

• Readme
• Docs.rs/sic crates
• https://github.com/foresterre/sic/wiki/Contributor-guide:-implement-a-new-image-operation
• Ask!