- make all of the following opt-in dependencies:
  - sqlalchemy
  - mako
  - pystache

- share the docs between README.rst and templatealchemy.api

- allow per-format rendering specs... eg:
    templatealchemy.registry.default.renderer = '{html: mako, text: mustache}'

- allow wildcard extmap, eg {'*': mako} ?

- handling of templates that don't exist is not so good...

- replace templatealchemy.util with aadict, asset, morph
