streamlineGlobalContext: Support resetting global context before handling request

In streamline, the global context will persist unless you reset it at the beginning of the request. This PR wraps the app.handle method to reset the context before continuing processing the request.

Edit by @aseemk: fixes #12.
Important: we don't want to overwrite the entire `context`, as that would lose things potentially set by other middleware. We just want to add to it.

(Which is the entire reason behind this feature — express-streamline now takes care of resetting the entire `context` at the beginning of every request.)

Minor: for clarity, maybe let's also use a different property name than `context`. E.g. `global`.