Metadata creation, index page, page alias edition.

 - When you attempt to read a file or to create/edit one, a json
   metadata file is created / read.
 - It contains 2 fields: index and aliases, index is the path of the
   index file, aliases is an object with the file names aliases.

TODO:
 - Alias can be inputed, and is transmitted to the server, it is even
   read server-side, but it isn't saved.
 - Metadata file is not commited when ran in git mode.