Feature suggestion: Exporting vertex colours if no material is present

[ArsThaumaturgis]

(As far as I see, GitHub doesn't seem to allow pull requests for specific commits. So I hope that it's okay that I'm simply creating issues for my commits, and linking to them to reference the changes. >_<)

This allows the user to export vertex colours when no material is present, as long as "force export vertex colors" is checked.

Commit reference: ArsThaumaturgis@9293f80

[ArsThaumaturgis]

It occurs to me that it makes sense to link the relevant "pull request" here: ArsThaumaturgis#1

(Note that it was made in my own fork in order to isolate the changes. My apologies if there was a better way to do so, given that I had already made the commits when I came to do this! ^^;)