Add support for human-readable times to the cacheControl directive

[mxstbr]

Integrating https://github.com/zeit/ms to allow userse to specify the maxAge of the cacheControl directive in a more human-readable way would be very nice:

type User @cacheControl(maxAge: 540) {}
# compared to
type User @cacheControl(maxAge: "9m") {}