Support for in-buffer variables

See updated README.md for details.
Merge pull request #17 from magnars/basic-support-for-vars

Support for in-buffer variables
Fixed crash when there're no variables
Prettify application/atom+xml as XML

Entities with content-type application/atom+xml were not prettified as XML, this should fix that.
Merge pull request #19 from vetler/patch-1

Prettify application/atom+xml as XML
Format application/atomcat+xml as XML

The application/atomcat+xml media type is described in RFC 5023, and should be formatted as XML.
Merge pull request #20 from vetler/patch-1

Format application/atomcat+xml as XML