Adding support for multiple feed formats

This is some initial support for adding Dash xml feed generation (#307). Note that support for specifying formats is a little crude and I've added a template for xml files which I think is a reasonable approach.

Basically, I've added a `--docset-feed-formats` which currently takes a series separated by commas with current types being `atom` and `xml`. and `--docset-xml-filename` to allow users to specify multiple feed formats and the corresponding filename to go with the xml feed. It might be better to just have a `--docset-feed-filename` and append the extension according to the format.

The other change is related to the `--docset-package-filename` which will now ignore the path extension and create the file with the appropriate extension depending on which format is being output. I realize this might cause issues if people are relying on being able to change the path through this argument but this seemed like the best way to do it without adding filenames for each format.

The xml file template allows users to create their own templates depending in case Dash or anyone else has a different format.

I'm not totally sold on how it's recognizing the feed formats but I think it's better than nothing.

This will default to Atom so that existing functionality will work.