plugin app manager: add a way to create files: a `files` field can now be used to create files on the fly. Its value must be a dict where key are file names, and values are an other dict with file data. For now file data can only use the `content` key. Value of content will be written to the file. If a file already exists, it is not overwritten.

You'll find install instructions in documentation (see doc/installation.rst).

If you need help, come the SàT official XMPP room: sat@chat.jabberfr.org or join us with a web browser via https://chat.jabberfr.org/converse.js/sat@chat.jabberfr.org