Basic support for Atom feeds

• Files with an `.atom` extension will be passed through the same
  X·S·L·T transform as `.xml` files, which now has Atom‐specific rules.

• Automatic generation for basic metadata. There is probably more work
  to do here to automatically pull things like titles and summaries.

Other small improvements added as a part of this work :—

• The `transform.xslt` file is now specified via a `$(TRANSFORM)`
  variable in the Makefile and can be overridden.

• The `xmlns:html` attribute is manually removed from the root element
  of outputted H·T·M·L (if it exists).

• Subdirectories are now properly created in the `public/` folder when
  needed.