Allow command flags to be used when constructing new message

[kwokfu]

NewMessage() now accept command flags.

e.g. to create a proxiable request

import { cmd, cmdFlag, app } from './diam/const.js';

diam.newMessage(cmd.CreditControl, app.ChargingControl, cmdFlag.Request | cmdFlag.Proxiable);

Introduce NewRequest() for message to have the R-bit pre-configured.

e.g. to create a request

import { cmd, app } from './diam/const.js';

diam.newRequest(cmd.CreditControl, app.ChargingControl);

Fixes #22.

[lwlee2608]

LGTM