feat: allow users to specify a propagation mechanism

[kjin]

Fixes #949

This feature allows users to supply a Propagation (or an absolute path to a file that exports that interface) to specify how trace context headers are injected and extracted from HTTP requests. It also replaces the built-in propagation mechanism with that of @opencensus/propagation-stackdriver, which has the same behavior.

Some notes:

• Some OpenCensus propagation libraries either don't export a consistent interface or have implementation bugs, so they won't work out-of-the-box as of yet.
• The propagation object exposed through the StackdriverTracer object doesn't have the same interface as the input object, the latter of which is modeled on OpenCensus while the former is just something I deemed to be more suitable for this module. Particularly of note is that OpenCensus treats all span IDs as length-16 hex strings, while we use decimal integer strings.
• It may be better not to accept an OpenCensus Propagation object because of the above reasons. I'm open to discussion on this.

[kjin]

Blocked by: census-instrumentation/opencensus-node#340

[ofrobots]

@kjin census-instrumentation/opencensus-node#340 is done. Is this still blocked?

[kjin]

It'll be unblocked when 0.0.10 is released. In other words, now blocked by Milestone #2

[emillg]

@kjin 0.0.10 has been released. Is this still blocked?

[kjin]

@emillg Thanks for the reminder. I'll move this forward shortly

[kjin]

I'm going to take a different approach on this... will leave the issue open so it won't get dropped.