Create the directories for the output target if it doesn't exist

[lewisf]

When using --output, if the desired output directory doesn't exist, we can make the directories automatically. This would make integration with other tools/processes easier

[alamothe]

Does --output take a directory or a file? Running:

apollo-codegen generate --help
# ...
  --output                                 Output directory for the generated
                                           files                        [string]
# ...

Suggests it is a directory, but it cannot handle anything other than a file (at east for TypeScript target)