Enable appending layers to motion sequences (to continue drawing a line after it's been started)

[Patabugen]

In response to this issue: https://github.com/Igor-Vladyka/leaflet.motion/issues/16

I've refactored the Sequence group a little so we can call addLayer() to add an animation step in the sequence after initialisation.

I've also added an examples folder as a form of documentation but also so I could use it for testing/developing this feature.

A couple of things to note:

• This is very bare bones and ready for me to try out for my own use case. Please let me know if you find any bugs, missing features or anything that doesn't work as you expect.
• There is one breaking change: I've changed seq.motionStart() to act the same way as polyline.motionStart() (to do nothing if the animation has started, instead of restarting the animation). It's not actually required for my new functionality.
• I have merged in my own pull requests into this branch, even the one which hasn't been merged into @Igor-Vladyka 's repo.

Example:

var trackPolyline = L.motion.polyline(trackRoute).motionDuration(2000);
var shipPolyline = L.motion.polyline(shipRoute).motionDuration(2000);
var carPolyline = L.motion.polyline(carRoute).motionDuration(2000);

// Build the Sequence Group:
var seqGroup = L.motion.seq([
    trackPolyline, shipPolyline
]).addTo(map);

// You can now call addLayer()            
seqGroup.addLayer(carPolyline);

If the animation hasn't started, the layer is just made ready, if the sequence has been started the new layer will either wait it's turn or start animating immediately if the existing sequence is finished.

[Igor-Vladyka]

Overall it's a good idea and a valid approach, but need more in-deept modification to SEQ and to the root motion module.

Please resolve conflicts, and merge it into separate branch.

[Igor-Vladyka]

I've integrated part of your changes into the plugin, and part of documentation for it. Please check it out, and test it.

[Patabugen]

Thanks @Igor-Vladyka ! I'm hoping to spend some time on the project using this in the next month or two so will have a look!

GitHub is saying some commits aren't merged, but I'm not sure how to see which they are - do you know how to? (I'm mostly curious to see which ones).