Add snakeOut() and more

[PitouGames]

Hello,
first of all, thank you for the nice plugin!

Here is my (big?) contribution to your plugin.
Maybe you should first look the demos I've updated (and the README).

It contains :

• Add your license, like in Various fixes #31
• Fix for performance.now() time resolution, like in performance.now() time resolution workaround #37
• Add snakeOut() function for both Polyline and LayerGroup
• Add snakeReset() function for both Polyline and LayerGroup
• Add an option to center the map on the snake's head, inspired by How to follow the path animation automatically? #19
• Update demos to include new methods/options and update to leaflet 1.5.1
• Update README to reflect changes

#22 can be closed, #29 can be closed
This PR include changes also made in #31 and #37, so I think they can be closed too.

[PitouGames]

Hello @IvanSanchez,
is this repo still supported?

[mattLLVW]

Hello @IvanSanchez , i too would be interested in these features.
Anything i can do to help speed up merge of this PR?