generate presentation in index.html, not presentation.html

The fact that the file is an HTML file is an implementation detail, and
should not be visible in the URL (who knows, we may need an index.php or
index.shtml in the future). By naming the file index.html, the public URL
of the presentation becomes the directory name, which is shorter,
simpler, and stable.
allow creating a project from the command line

Dahu was able to open an existing project from the command-line. After
this commit, if the project passed on the command-line doesn't exist, it
is created.