Upgrade to version 1.3.1

- new Composer scripts to clear cache at install/update
- MarkdownExtended configuration with internationalization strings
- new MarkdownExtended "toString" features (replacing the old "_html")
- new "printers" responsive class (and stylesheet) ; update of
templates to use this new feature ; new printer-specific footer
- the content tools are now "responsive" (shown as a menu in tablets
and smartphones)