Add support of menu

[Psycojoker]

Regarding menu.html I think that we should use template inheritance to avoid code duplication between index.html and menu.html http://jinja.pocoo.org/docs/dev/templates/#template-inheritance

[Psycojoker]

I think that we should also have a decorolation between menu displayed text and file name, something like:

- about: "Qui sommes nous ?"

instead of:

- "Qui sommes nous ?"

This will reduce the chance of breaking link because someone decided to change a menu title and the chance of generating bad file names (to be sure to avoid this problem we should slugify the text).

[Psycojoker]

(for reference)

[Psycojoker]

Also, I think that:

• the menu should only be defined in the root settings.yaml (it can be transmitted to other pages)
• I would not add the home: Home key and actually let the user do that by hand if they want to
• and menu_title: "Menu 1" can be sent to the template since we know which one we are generating, so no need for the user to explicitly put it here

Oh, and one last thing, would you have the courage to ... update the README.md /o\?

I hope all those comments aren't too much :/

Oh and do try to merge with my master branch, I've made some heavy code modifications, I'll handle the merge myself ;)

And thanks a lot for this PR, it's a really cool feature :heart: