Provide plain text alternative to HTML message

[fmichonneau]

I saw that #62 raised the issue of plain-text alternative for use with RStudio Connect but I'm wondering about using [multipart/alternative](https://en.wikipedia.org/wiki/MIME#Alternative) more generally as a way to provide a plain-text fall back for email clients that do not support HTML content.

It looks like a lot of the mechanic to make this work is already part of the package.

[jcheng5]

Were you hoping to be able to explicitly provide the plaintext content, or for blastula to automatically dumb down the HTML to plaintext for you?

[fmichonneau]

I'm writing my emails in markdown, so providing the raw markdown as the plain-text version of the email would be great but I'm not sure how that would generalize to other use cases of blastula.