Support for Facebook Messenger `element_share`

Botman should support the `share_contents` field for the button of type `element_share`.
Here's the facebook documentation: https://developers.facebook.com/docs/messenger-platform/reference/buttons/login#example_body
and here an example of the payload:
```
"buttons": [
{
"type": "element_share",
"share_contents": {
"attachment": {
"type": "template",
"payload": {
"template_type": "generic",
"elements": [
{
"title": "I took Peter's 'Which Hat Are You?' Quiz",
"subtitle": "My result: Fez",
"image_url": "https://bot.peters-hats.com/img/hats/fez.jpg",
"default_action": {
"type": "web_url",
"url": "http://m.me/petershats?ref=invited_by_24601"
},
"buttons": [
{
"type": "web_url",
"url": "http://m.me/petershats?ref=invited_by_24601",
"title": "Take Quiz"
}
]
}
]
}
}
}
}
]
```