[feature request] Implement since and until attributes

[evlist]

I am using this plugin to track multi-days hiking tours with PhoneTrack and to use the [tsmap] shortcode I haven't found a better solution than configuring the plugin to generate a new track every day and adding a pre-generated list of track ids in the track attribute.

This is working because the validate_track_ids() methods only keeps the ids which exist and are valid and silently ignore the others but I don't find this obvious to set up and I suggest to add a couple of attributes (that could be since and until) which would select all the valid tracks between these two dates.

You could then set the since attribute to the start date for your tour and add the until date when it's over so that the map keeps displaying the tracks related to this tour.

Note: I will be leaving for a new hiking tour in a couple of days and won't have enough time to implement it right now but if you find this suggestion useful I can propose a PR when I'll be back.