[RFC PATCH] Allow end user to know what the original id of a feature was

[taylor.smock]

There are two options:

1) Set the id of the object to the "original" id
2) Add a tag with the "original" id. This would have to be a "well-known" tag to avoid other people using it (current_id probably isn't a good choice, maybe xml_current_id?).

For the attached patch, I did the latter. There are no unit tests yet (I don't want to change them if the implementation uses (1) instead of (2)).

[taylor.smock]

Implementation of (2) (no unit tests)

[taylor.smock]

Add generic unit tests, with a test that will fail until a strategy is decided upon (at which point it will need to be modified)