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.
Playlist URL replacement (and refact for reuse)
Fixed some of things that I reviewing. Did not test however.
Minor fixes to the fixes
Merge pull request #1 from ozinc/ddTagsSegmentReplace

Great, thanks - tweaked only a little bit.