DirectoryEntryReference: implement chapter 3.2.6

This commit adds in the implementation for RFC5545's chapter 3.2.6.

Moving forward, there are two TODO's that should be considered:

  1. When the CAL-ADDRESS type is implemented, there needs to be
  something added to this and other classes like it that enforce the
  restriction on which content line types are allowed to possess this
  parameter type.

  2. When this project is able to be ported over in such a way that it
  can be browser packaged, there might be an issue with using the NodeJS
  implemented URL class. I'm not sure about if it will work or not, but
  future me looking at git blames for why it is not working should take
  some direction from this message. Keep going, future me, keep going!

Signed-off-by: Mark Stenglein <mark@stengle.in>
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