Add support for httpStatic middleware

[knolleary]

• New feature (non-breaking change which adds functionality)

This is an alternative proposal to #4125 that appears to have stalled.

Rather than add httpStaticMiddleware as a top level property, this follows the lead of #4109 to support a middleware property on the httpStatic object:

httpStatic: [
    {
            path: '/opt/static/',
            root: '/private/',
            middleware: myCustomHttpMiddleware
    }
]

[Steve-Mcl]

Approved (looks sane)

Do you want me to pull and test on a windows box Nick?

Happy either way.

Will leave for you do decide / merge.