varargs(args, offset, ...extra_args) {
				return extra_args.concat(Array.prototype.slice.call(args, offset));
			}
Extract all values from the given argument array beginning from
`offset` and prepend any further given optional parameters to
the beginning of the resulting array copy.

@memberof LuCI.baseclass
@instance

@param {Array<*>} args
The array to extract the values from.

@param {number} offset
The offset from which to extract the values. An offset of `0`
would copy all values till the end.

@param {...*} [extra_args]
Extra arguments to add to prepend to the resulting array.

@returns {Array<*>}
Returns a new array consisting of the optional extra arguments
and the values extracted from the `args` array beginning with
`offset`.