Make .libcst.codemod.yaml location an optional argument for CLI usage

This way you do not need to run LibCST from the same directory in which you have the configuration. This is helpful if you want to commit a per-team LibCST configuration into a monorepo and not conflict with other teams' LibCST configurations (which also might need to be in that same top level directory).

This also means that it becomes easier to package the configuration alongside codemods with a build system and create your own per-team CLI.