config: Use extended interpolation in Config

This avoids triggering accidental interpolation errors when your URL
contains percent signs (e.g. %2F).  Curly braces, on the other hand,
will never appear in an encoded URL.  From RFC 1738:

  Unsafe:
  ... Other characters are unsafe because gateways and other transport
  agents are known to sometimes modify such characters. These
  characters are "{", "}", "|", "\", "^", "~", "[", "]", and "`".

  All unsafe characters must always be encoded within a URL.

Signed-off-by: W. Trevor King <wking@tremily.us>