Allow downloading of sequences in BED/GFF(3) format

[blakesweeney]

It would be useful to allow users to download results of a search in BED/GFF(3) format. We will have to do something to inform the users that they are only downloading a subset of the results (those that have coordinates) so people don't get too confused. We could:

1. Disable download unless all results have coordinates.
2. Show a warning on the download page.
3. Help tips over the format to show the message.

and probably other stuff. I'm inclined toward the second option.

Note that before we do this we should probably fix #123 before doing this.