Set up extlinks extension

Configure the extlinks extension to make it easy to link
to issues from the documentation.

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>
Ignore missing extensions in named dispatch

If the NamedDispatchExtensionManager is told to invoke
and extension that it has not loaded, log the condition
but otherwise ignore it.

Resolves issue #14

Signed-off-by: Doug Hellmann <doug.hellmann@dreamhost.com>