Add support for compressing arbitrary URLs.

[davidlehn]

Add support for compressing arbitrary URLs. Any id which is in the appContextMap will be compressed as a single number before using other URL codecs.

• Need feedback if this is a good feature and approach.
• The appContextMap naming is being overridden here and elsewhere. This feature could change and use another map.
• The sub-codec is value is important. 1026 is encoded as 3 bytes. A low value would be 1 or 2 bytes. If lots of URLs get encoded this would add up.
• This was modified from other codecs, it may be able to be cleaned up if not handling fallback cases.
• Needs tests.

[dlongley]

I'm closing this because PR #87 supersedes it.