Add py:else directive

[hodgestar]

On the Genshi mailing list Eoghan Murray suggested the addition of a py:else directive that is nested inside the py:if directive and is an alternative to py:choose for simple binary output choices.

E.g.

>>> tmpl = MarkupTemplate('''<div xmlns:py="http://genshi.edgewall.org/"
...   py:if="False">
...   Will not be output
...   <py:else>Will be output</py:else>
... </div>''')
>>> print(tmpl.generate())
<div>
  Will be output
</div>

[hodgestar]

Initial outline of implementation (from Eoghan's email with a little clean up by me).

[hodgestar]

A few things are needed before this can go in:

• Tests for all the common cases.
• Double-check whether any further documentation updates are needed.
• The current implementation omits the element with the py:if attribute when handling the py:else case. I'm not sure whether this is right or not (I can imagine use cases that for both options).

[hodgestar]

Moved to milestone 0.9.