Add disableMobile and enableMobile public functions

[nyroDev]

Following this issue:
#675

I had a case when a mobile menu had to scroll because it was too long.
With skrollr, all event where stopped and it was impossible to scroll in the menu.

With this pull request, we can now disabled all event catching on mobile.

The usage is simple:
When the user open the menu, I call skrollr.disableMobile() and it can now scroll in the menu, and skrollr stay where it is
When the user closes the menu, I call skrollr.enableMobile() and everything work like normal.

I updated the README.md to add these 2 functions documentation.

I didn't updated the minified file, neither the HISTORY.md.

I hope it fits the project.

[timawork]

Hello, I try it
jQuery('#openmenu').click(function(){
skrollr.disableMobile();
});

but

Uncaught TypeError: skrollr.disableMobile is not a function

[timawork]

Ups sorry, my fault. All works perfect

[manooog]

I also get

Uncaught TypeError: skrollr.disableMobile is not a function

how do you fix it ?

@timawork

[timawork]

This works for me

var s = skrollr.init({
.....
});

jQuery('#openmenu').click(function(){
s.disableMobile();
});