Make deftemplate and opt-ask public

[pepe]

Closes #221

[sogaiu]

Though I don't quite follow all of the details, here's an attempt at a version of one of the docstrings with a few tweaks:

Define a template inline in the manner of Python PEP292 (shell-like $ substitution), also allowing dashes in identifiers.

A new function template-name is defined that takes a dictionary opts containing substitutions as key-value pairs. Keys should be keywords with names that correspond to those of the substitution names.

The template is parsed from body which should be a string and can contain substitutions.