non-blank default script_path value prevents Timeline from computing correct value

As reported in Zendesk 9021, a locally installed distribution of TimelineJS does not retrieve font.default.css from the local distribution.

In short, this is because Timeline comes with a default value for script_path as seen in
https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/timeline/Timeline.js#L99

This means that later, TimelineJS will not compute a proper relative script_path:
https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/timeline/Timeline.js#L169-L171

The workaround is to explicitly set script_path in the options passed to the Timeline constructor on a page using a locally served distribution. The explicit value should be a relative URL to the "js" directory of the distribution, eg https://example.com/timeline3/js/

Our intention is to support an undefined script_path option, where TimelineJS "introspects" the location of the file where its own code was loaded and uses that. Users should only need to define script_path if the TimelineJS code is bundled together with other JS resources so that the loaded bundle isn't located in the right place relative to the other resources (CSS and language localization files, generally) which may need to be loaded.

It may be as simple as blanking out the script_path value at https://github.com/NUKnightLab/TimelineJS3/blob/master/src/js/timeline/Timeline.js#L99 but of course that should be thoroughly tested.

Start in src/js/timeline/Timeline.js at the default script_path near line 99 and the path computation near lines 169-171. Verify the behavior with a locally served distribution, including loading font.default.css and localization resources. Done means an undefined script_path computes the distribution's relative path while an explicit value still overrides it.