Load all files in a data directory: "_all.yml" and other files if present.

[PaulPichaureau]

Sometimes it could be interesting to have a "_all.yml" files for many little pages and some file for big pages.

I propose to load all the files, plain and simple. :-)

[jayvdb]

I've created https://github.com/thanethomson/statik/pull/89 and https://github.com/thanethomson/statik/pull/90 with the two enhancements in this PR. As far as I can see, that is the only two enhancements in here, that are not already merged.