[Proposal] Add text/plain part to MailMessage

[igorsantos07]

The MailMessage class has a very similar API to Mailable, except it's simpler to use (and thus, is sampled on the Notifications guide).
However, it's missing an important part of emails for deliverability: the text/plain part.

Sure, it can be generated using the Markdown parser, but it becomes something not easy to be done when you need to use custom templates.
Mailable solves that by having all three methods: view(), text() and markdown(), but the lack of MailMessage::text() troubles some users (me included).

[taylorotwell]

I suggest returning a Mailable from your notifications toMail method.