Backup the todo.txt file on every modification

[jmanuel1]

At present, the file as backed up only when the plugin is activated. To protect against data loss due to file corruption, the todo.txt should be copied regularly instead. I propose copying after every modification.

Also, maybe I should look into a better way to asynchronously read and write from a file safely. If I can do 'the right thing', then I might start actually using the plugin.

This issue refers to the following TODO:

todoer/lib/main.es6

Line 60 in e759e39

// TODO: Back up on every modification

[jmanuel1]

Google Drive, where I store my todo.txt, keeps previous versions of the file for up to 30 days (unless I choose to keep a revision forever). I think this is good enough for me, so I will deprioritize this issue.