Add ability to exclude functions (eg when using Container Images)

[AnalogJ]

When a function image is provided, the go plugin should not attempt to compile code (as the handler key is missing).
https://www.serverless.com/blog/container-support-for-lambda

an Exclude key, similar to the newrelic plugin, would be ideal.

[OranShuster]

We have a mix of containerized and compiled go code. I don't see the plugin trying to compile our containerized functions
can you share a config of a function that is being compiled on deploy/package?

[mthenw]

Fixed with the above PR