Fix issue #256: Copy-pasting from Notepad does not work

[YoranBrondsema]

A PR that addresses #256. Fallback to text/plain if text/html is empty.

[bantic]

Thanks! As written, this continues to use the HTMLParser to parse the plaintext, so if someone copied the text "abc <b>bold</b>" it would get pasted as "abc bold". Ideally there would be a text parser in addition to the HTML parser, and it could interpret pasted plaintext into mobiledoc (also splitting into multiple sections by linebreak, for instance).
I'm going to merge this and add an issue to use an actual text parser. If you have interest in tackling that you would be welcome to, otherwise I should get to it sometime this week.
thanks @YoranBrondsema !

[YoranBrondsema]

Ah true, hadn't thought of that case. Well at least we have a semi-functional plaintext pasting functionality now so thanks for merging! I'll see what I can do this week, I might find some time to tackle this as well. I'll let you know in #263 if I start working on it so we don't do double work!