Support custom file extension for page filename

[baco]

It would be useful to have a config option that allows to customize the Page file extension, for example allowing Page files to have a generic .txt extension.

Use case: I already have a directory with a lot of Git version-ed .txt files with MarkDown markup in them, that I used in a previous documentation system. I now want to migrate them to a wiki system (Gitit), and it would work out-of-the-box but first I should git mv all files from .txt to .page. This is rather uncomfortable.

[wcaleb]

This duplicates #298. There is a pull request to partially address it at #445.

[wcaleb]

Resolved, I believe, by 56e9d7a. Could probably close this.