Support ordering pages and articles by metadata attributes and sorting functions

[davidmarble]

This fixes #420.

This change makes the default sorting order for pages their filenames (new). The default sorting order for articles is still by slug (same as before). But you can tweak how either is sorted in your pelicanconf.py. For example:

PAGE_ORDER_BY = 'page-order'

In this case, just make sure that all pages have the metadata attribute 'page-order'.

Advanced: You could also use a sorting function. See the code for an example, as a sorting function is used to get filenames based on the source_path attribute of content.

[justinmayer]

Hey David. Thanks for the contribution. Travis is reporting test failures; did the tests pass when you ran them on your system, as documented in the How to Contribute docs?

[davidmarble]

Whoops! I'll take a look, fix, and re-submit.

[davidmarble]

Those are errors in the existing master branch.

[justinmayer]

How are you reaching the conclusion that there are test failures in the existing master branch? I'm not seeing that.

I am, however, seeing test failures related to this pull request

[avaris]

@davidmarble , I don't think this is correct. You are changing the key that is used for grouping/gathering translations, and this would lead to weird consequences (not to mention the current test error). Suppose I gave ARTICLE_ORDER_BY = 'author', this would result in marking every article with same author as translations of each other.

If you want to change the order of articles or pages, you should do it after process_translations is completed.

[davidmarble]

Spent some time getting tests to pass on the existing master branch for Windows before coming back to this. Just submitted a pull request for that work.

I see what you're saying now about translations. I hadn't yet tried any translation work. Now that I have tests passing for master on both Windows and Linux, I'll consider some other ways of going about this feature.