header-parser: add reviewer

[elibarzilay]

Related to DefinitelyTyped/dt-mergebot#248: when this was discussed, the conclusion was that there's no good way to guess when an owner approval is required, and therefore the header should be extended with an optional field for the required approval level. The value should be owner or maintainer.

(Maybe for completeness allow other too? The parser can just ignore the actual value, and leave that for the mergebot to decide what to do with it.)