add a config value for time format

[Leonardo J. Caballero G.]

It would be great to choose the time format used for this sphinx extension, as the format for a "datetime strftime" call.

For example, from Sphinx project config file (config.py) using this extension with a new config value:

recentpages_sidebar_fmt = '%d de %B de %Y'

or

recentpages_sidebar_fmt = '%d/%m/%Y'

That generate a specific 'datetime' format like this '%d/%m/%Y' (03/12/2013) instead of this default format '%Y-%m-%d'.

I can take care of this task.

I created a fork of this repository for work this issue.

[Sho Shimauchi]

Thanks Leonardo! It's a good idea :) Please send me a pull request and then I'll check the patch.