Fix multiple localSchemaFile bug

[JakeDawkins]

Because of how oclif handles flags, it was possible to incorrectly parse a command with multiple --localSchemaFile flags like the following:

apollo client:codegen --target=swift --localSchemaFile=schema.graphql API.swift

This command would parse the last <output> argument as a schema and fail to parse. This is a bug that currently exists in oclif with how multiple: true flags are parsed.

oclif/oclif#190 (comment)

This PR changes flag parsing to only accept a single flag for localSchemaFile, allowing for a comma-separated-list of schema files like:

apollo client:codegen --target=swift --localSchemaFile=schema.graphql,schema2.graphql API.swift`

TODO:

• Update CHANGELOG.md* with your change (include reference to issue & this PR)
• Make sure all of the significant new logic is covered by tests
• Rebase your changes on master so that they can be merged easily
• Make sure all tests and linter rules pass

*Make sure changelog entries note which project(s) has been affected. See older entries for examples on what this looks like.