Custom datadog tags and Segment host replacement

Initial support for custom datadog tags in probe and template config,
using the ddTags property.  If configured on both, what’s configured on
probe overrides the other.

Support for configuring a replacement hostname for fetching Segment in
the VX probe. Configured with property vxSegment.
Nothing major but its actually enough here to just say "status: res.statusCode". And there seems to be some tradition in JS to only use the commas when you need it (for example for keys that have hyphens in them, such that x = { 'stuff-x': 1337 }).