add zoom in/out feature

[re7w12u]

Hi,
I've been using your module (thanks btw ;) ) in an application and I wanted to leverage the Vis zoomIn zoomOut feature. This leads me to 2 questions :

• I was able to make it work, but I had to change the node_modules/@types/vis/index.d.ts and add 2 new method signatures in the Timeline class (line 723). I can feel this is kind of wrong. I'm fairly new to this and I'm not sure about how to do it right. What would you suggest?

• Once it's properly done, would you agree on merging my code?

[re7w12u]

I've also found another way to get the feature by implementing the code directy in the service. see my last commit.
This way, no need for index.d.ts...

[hypery2k]

@re7w12u are you still interested in adding this feature?

[re7w12u]

@hypery2k Yes, for sure ! Let me know if I can help

[hypery2k]

if you like you can sent a PR to my fork.