Add a way to deactivate type bindings for a specific canister in dfx.json

[rvanasa]

This feature request is motivated by dfinity/motoko-dev-server#43 along with frequently running into this myself. While it's possible to run dfx generate foo, projects with a large number of canisters would benefit from a way to exclude specific canisters in the dfx.json configuration.

One approach could be to skip binding generation when declarations.output is null (which currently uses the default declarations/* path) in the dfx.json configuration. Another option would be to add a new optional field (maybe declarations.skip or similar) for complete backwards compatibility.

Thoughts?