0.12.6

feat: Use builder pattern and log unhandled notifications (#84)

Potentially fixes #83

Prior to this, any unhandled notification not starting with `$` would
have caused the main loop to exit with error resulting in server to
crash. Moreover, this unhandled_notification handler only works with
builder pattern which imo is cleaner and offers ability to separate out
implementation of various requests into its own files (i'll maybe do
that someday).