reduce required dependencies

[HookedBehemoth]

• move env_logger to dev-dependencies (was only used in the examples)
• move image functions behind existing feature flag

I bumped the version to 0.7.0 as this could be a breaking change for someone having appctx but not image enabled.

The motivation was to flatten the dependency tree, reduce build time and reduce target folder size.

[bkirwi]

Seems reasonable enough to me! I'm not aware of any real-world applications that use appctx, but it still seems wise to bump the version.

@fenollp / @LinusCDE - unless you object this seems mergeable?

[fenollp]

LGTM!