Export notification state to file for use with systray

[cfcs]

Generates some warning messages because of the Lwt.bind-freakshow going on in the file writing function. I've still no idea how Lwt is supposed to be used, but this works for my purposes. Rewrites welcome :-)

[hannesm]

while I appreciate this contribution, I'm a bit uneasy with its current implementation (using Lwt.join and spreading all over). Instead, I'd do the following (and will if time permits):

• use a React.event (or signal) for notifications
• have a listener via a channel that waits on this (or a second signal) and when something arrives, writes to a file (inherently not racy since there is only one listener!)

This would cleanup the mutable notifications, and move the file writing into a single place.

[hannesm]

closing now that #55 (or #56) hit master. thanks!

[cfcs]

Awesome!