Add support for human-readable times to the cacheControl directive

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:

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

First inspect the cacheControl directive's maxAge handling; the issue names no file or test. Done means the directive accepts human-readable values such as "9m" alongside numeric seconds, with coverage for the documented examples.

Written by the indexing model from the issue text.