Make the cacheBuster key definable by setting an options

[frewillems]

HI guys,

I'm currently working in an environment where we have to specifically allow get parameters. We are behind Akamai and they ignore all not allowed params. We have a company wide standard for our cacheBuster keys, 'c'. So I was looking for an option to set the key, and I was surprised to see this is not possible.

What about changing options.cacheBuster so it can contain a string. So when this option is undefined, we default to 'z', when false we default to '' and when actually defined, we use the provided string.

[alvinlin123]

Sounds like a good idea, feel free to submit a pull request or I can make it happen when I find some time.