Allow setting per-template content negotiation defaults

[fluffy-critter]

Expected Behavior

As a followup from #157 / #587, please allow the site operator to designate which templates should take precedence when there's no Accept: header provided.

Current Behavior

The current priority ordering is super arbitrary.

Possible Solution

Provide a priority mapping for extensions on a per-template-name basis, e.g.

config.template_default = {
    'feed': ['.atom', '.xml', '.html'],
    'calendar': ['.ics', '.html'],
    'index': ['.html'],
    '': ['.html']
}

Which is of course only meant to be an example, since generally only one or two templates will have content-type negotiation or multiple renditions to begin with.

Context

Several RSS-consuming clients (e.g. feed readers and the MonitoRSS Discord bot) do not send an Accept: header, and defaulting to HTML leads to incorrect behavior.