Add versioning

[fvisin]

It would be amazing to have versioning, i.e. to have a history of each note and to be able to revert a note to a specific past version. A way to implement it could be either to save a new version of each note whenever another note is opened and/or when the user leaves the website (e.g. when the session ends?) or to add a "Save" button and enforce the user to save the note before opening a new note or leaving the page (arguably not as comfortable as autosave).

This might be related to https://github.com/nicbou/markdown-notes/issues/2 as it makes a reference about the history of the note, so there might already be something similar to what I am proposing under the hood.