Rstrip remote output

Keeps every response from having an extra line.
Missing tag_updater import
Better Reader plugin capabilities.

The big change is that attributes "edit_" attributes of ReaderPlugin
derived classes will get (body, extra_content) passed to them and are
expected to return (body, extra_content) edited.

Body is the HTML component from the item (likely the summary). Body
content will be fed through the HTML parser, and links embedded into it
will be properly handled by canto's reader (for display options, and
goto, etc.)

Extra_content is naturally formatted and can have canto theme escapes
(%B %1 %[56] etc.) in it. It's displayed after the item content.
Try harder to list enclosed media in Reader

- Had to actually request the content, first

- Now detect ['enclosures'], ['media_content'], and ['links'] with
  relation 'enclosure'
Add reader-extras plugin

This is geared toward adding unobtrusive extra information to the reader
output. I think these should mostly be enabled by default, but
considering it's compiling regexes etc. perhaps not.

In addition, it has a DEBUG_CONTENT option that will append all
available information to any item opened in the reader. Useful for
finding out what neat extras you can expose.