Add generic-names hashPrefix option

[pascalduez]

Hi,

this PR allows to pass the hasPrefix option to generic-names directly, easier than building our own generateScopedName function and using that lib anyway.

Similar to Webpack css loader (undocumented) hashPrefix query parameter.

[pascalduez]

Ping @sullenor :)

[sullenor]

I haven't thought that someone will ever need it :)
Anyways, since I exposed it in generic-names let's add it to the api.

Thank you for your efford

[sullenor]

4.0.3

[pascalduez]

Thanks @sullenor.