Add support for default values for function arguments

[jaapz]

This is valid es6:

    let myFunctionWithDefaults = function(someNormalArgument, 
                                          argumentWithDefaultValue='default') {
         ... function body ...
    }

But the argument with default value is not highlighted for me. Could you add support for this syntax?

[othree]

I think this is already in.

[jaapz]

@othree Well it does not work for me, does it work for you? I have yajs.vim installed through vundle.

[othree]

Maybe conflict with other stuff, or you are not on latest reversion?

[jaapz]

I just checked, and I am on a the latest commit in the master branch. Is there any way I can make sure the yajs.vim syntax file is actually used? Although I'm pretty sure it is.

[jaapz]

I'm sorry, turns out I did have an extra javascript syntax file that was being used instead of yajs...