reorganize default template functions into class
reorganize default template functions into class

Using a class wrapper allows additional context to be provided to the functions.
Namely, we can now provide the Item itself to the function, so %foo{} could
conceivably do something useful even without arguments. This will be used for
the upcoming %unique function.

google-code-export/beets,google-code-export/beets,google-code-export/beets