Added pretty_pages setting

In the current version of Jekyll, the only way of stripping the `.html` extension from the pages (not posts) was to have the setting `permalinks: pretty`.  To solve this problem, I added a new parameter to the configuration file: `pretty_pages` (which, by default, is `false`).

With the proposed changes, for the page `page.html` there are 3 possible options

1. `permalinks:pretty` and any setting of `pretty_pages` :arrow_right:  `/page/index.html`
2. any setting of `permalinks` different from `pretty` and `pretty_pages: true` :arrow_right:  `/page/index.html`
3. any setting of `permalinks` different from `pretty` and `pretty_pages: false` :arrow_right:  `/page.html`

I updated also the documentation to reflect the changed behavior.

As a side note: as now there are two different ways of producing the same outcome, I basically duplicated a few tests, because they check two different settings. I don't know if this is 100% correct.

This closes #1566