Multiple generated files

**Is your feature request related to a problem? Please describe.**
I'd like to generate independent files for different schemas/operations so that all the generated code won't end up in a single file.

**Describe the solution you'd like**
Let the user specify multiple paths in the `generated` section.

**Describe alternatives you've considered**
Multiple `genqlient.yaml` configs?..

**Additional context**
Not needed.

The issue names no files or tests. Start by tracing how the `generated` configuration is parsed and how schemas or operations are assigned to output files; done means users can specify multiple paths and independent generated files are produced.

Written by the indexing model from the issue text.