TiddlyFox 2.0.1 only works if browser history enabled

[cd2357]

When running Firefox with "Never remember history" configured, the TiddlyFox icon does not appear in the toolbar and no automatic saving is possible anymore.

When switching the browser back to "Remember history", the TiddlyFox icon comes back.

If the browser history being enabled is a prerequisite for TF, please add a note in the plugin description or have the plugin show a warning if the history is disabled.

Running Firefox 52.0.2 on OSX.