imageproc: add conditional compilation feature

[foresterre]

Adding #335 will pull in the full imageproc and rusttype crates.
I would like to add them behind a conditional compilation feature.

To be precise, I would like to add the following optional dependencies and feature to the sic_image_engine crate:

[dependencies]
imageproc = { version = "...", optional = true }
rusttype = { version = "...", optional = true }

[features]
imageproc-operations = [imageproc, rusttype]

Now, if I were to provide the --all-features flag, the imageproc-operations should be set, however if called from the root of the workspace it isn't. I rather not add the same dependencies to the root workspace crate in order to keep the dependencies clean and localized.

Goal: figure out how to enable conditional compilation in a sub crate and propagate the conditional from the root of the workspace

[foresterre]

implemented by #410 (see also #395)