Add support for French braille

[bertfrees]

• Test the Liblouis based translator for French text and improve if needed. Note that the table has already been improved quite a bit lately.
• Provide a style sheet that implement the French formatting rules. The style sheet should ideally be modularized which should make it easier for users to customize the layout. Alternatively we could work with options to enable or disable certain features.
• Write tests for the Liblouis table (Liblouis YAML tests) and the style sheet (XProcSpec). Note that there are already a lot of Liblouis tests.
• Write some documentation: available options, special class names, recognized meta data, etc. The CSS source code and the XProcSpec tests also count as documentation. They should ideally have comments with references to the official braille code specification document.

We'll base the work on the official French standard (Braille Français uniformisé): see https://github.com/liblouis/braille-specs/blob/master/french/README.md.

Any other specific requirements that BrailleNet has should be handled with custom extensions to the braille table and/or style sheet.

[bertfrees]

The requirements have been documented on the wiki. Thanks to Yilin.

[bertfrees]

@YLBrailleNet Good job with the requirements document! Greatly simplified compared to the official document but still well detailed. And the mapping to DTBook is very useful.

Not everything is possible with the current version of Liblouis and the current CSS support, but it is for the most part. I suggest we look at the few problematic requirements after everything else is implemented.