Implement proposed command APIs in package twicmd

[diamondburned]

The proposal defines the following Protobuf:

message Command {
  // The start of each slash command. Each service registers its own
  // single-word name, usually the brand name, to be used in this.
  string service = 1;
  // Command arguments, in the format key:value. This must be immediately
  // after the service name.
  repeated CommandArgument args = 2;
  // The trailing argument, in the format value. This particular argument
  // requires no quoting.
  optional CommandArgument tail = 3;
}

message CommandArgument {
  string key = 1;
  string value = 2;
  optional string description = 3;
  // The hint that the parser will use.
  // The NLP will also use it to determine speech recognition.
  CommandArgumentHint hint = 4;
}

enum CommandArgumentHint {
  COMMAND_ARGUMENT_HINT_NONE = 1;
  COMMAND_ARGUMENT_HINT_STRING = 2;
  COMMAND_ARGUMENT_HINT_NUMBER = 2;
  COMMAND_ARGUMENT_HINT_INTEGER = 4;
  COMMAND_ARGUMENT_HINT_PERSON = 5;
  COMMAND_ARGUMENT_HINT_EMAIL = 6;
  COMMAND_ARGUMENT_HINT_PHONE_NUMBER = 7;
  COMMAND_ARGUMENT_HINT_ADDRESS = 8;
  // ...
}

We should implement this in twicmd as actual Protobuf code then connect the existing slash commands parser package to it.