Add a method for in-place generating a default Procfile

[josegonzalez]

Description of feature

While this isn't useful outside of the Dokku context, it would be great if we could generate a default Procfile based on whatever language was in use. If no Procfile exists, we would generate a default one based on the following template:

web: function.handler

The function.handler would be different depending on the language, but would be defined as follows:

• dotnet: test::test.Function::FunctionHandler
• go: bootstrap
• nodejs: function.handler
• pip: function.handler
• ruby: function.handler

We could get smart and detect the function filename from lambda_function.$ext as well.

[josegonzalez]

I wonder if we could instead set this as the default cmd? 🤔

[josegonzalez]

If a file isn't detected, then we would default to... Nothing? I think that makes most sense.

It might also be cool to specify a --handler argument that would override anything autodetected. This --handler would also be set as the default CMD in the generated docker image from #7.