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.
There is actually a package for this that is provided by-default by NodeJS so we don't need to add a 3rd-party dependency here: 
```javascript
var urlutil = require('url');
```
And it seems to work the same as uri-js!