Add support for Twilio SMS outgoing media attachments (MMS)

[krismuniz]

Hello Botkit team 👋

PR Summary

This PR adds support for Twilio SMS outgoing media attachments (MMS) along with documentation on how use the feature.

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Sending media attachments (MMS)

To send media attachments, pass a mediaUrl property to any of Botkit's outgoing messages functions (reply, say, ask, etc.) with an optional text property for text that goes along with your attachment.

/*
  Sending an attachment
*/
bot.reply(message, {
  text: 'Optional text to go with attachment',
  mediaUrl: 'https://i.imgur.com/9n3qoKx.png'
})

Note: your Twilio number as well as the recipient's phone must support MMS for media attachments to work

For more details on outgoing media attachments and a full list of accepted MIME types, go to Twilio's docs on media attachment.

[benbrown]

this field should be called text - it gets renamed during the send process

[krismuniz]

Oops, I missed that. Fixed!