Adding ability to read/write Permission Configs and cli to access them

- Added the ability to set/list/remove permission configurations from prefixes.
- Most of this was a mimic of what Spencer wrote for the zone configurations.
- Also consolidated some of the repeated code where I could.

Once this is merged, I'll create a similar treatment for Accounting Configurations.
How about we add a new function to subtract these lines:

  // sanitizeYAML filters lines in the input which match xxx_unrecognized, a
  // truly-annoying public member of proto Message structs, which we
  // cannot specify yaml output tags for.
  // TODO(spencer): there's got to be a better way to do this.
  func sanitizeYAML(b []byte) []byte {
    return yamlXXXUnrecognizedRE.ReplaceAll(b, []byte{})
  }