Add support for key=value pairs in annotations

[mihaibudiu]

This would enable annotations like

@pkginfo(name="my package", date="2018/3/29", version=2)

[ChrisDodd]

duplicate of #99 ?

[mihaibudiu]

Seems to be. But the key=value part does not seem to be implemented...
So we should probably reopen that.

[ChrisDodd]

IIRC at the time it was decided that full java-style identifier = expression annotations were not needed. But they should be relatively easy to add; the main concern is not impacting existing uses of simple expression annotations.