Use more XDG folder for default directories.

[saro]

At this stage we are using as default directory .notes to store the text notes and afterwards the exported html files. While on one side this is convenient to access quickly the folder, on the other it will contribute to pollute user's home directory with more hidden directories. Possible alternative is to use XDG folders[1] and in particular $XDG_DATA_HOME which would fallback on ~/.local/share/. Here we could create our vim-notes folder and let the user store his files there.

Needless to say, this will make a bit harder the directory to be reached but in autocompletion wetrust :)

Let me know what you think about it.

[1] - https://ploum.net/207-modify-your-application-to-use-xdg-folders/