Add parameter for custom property-path delimiter.

The default property-style output can be ambiguous:

  HashDiff.diff({'example.com'=>{'foo'=>'foo'}}, {'example.com'=>{'foo'=>'bar'}})
  => [["~", "example.com.foo", "foo", "bar"]]

This patch allows the user to provide a custom delimiter:

  HashDiff.diff({'example.com'=>{'foo'=>'foo'}}, {'example.com'=>{'foo'=>'bar'}}, "", 0.8, "\t")
  => [["~", "example.com\tfoo", "foo", "bar"]]