BinaryPropertyListParser/Writer: Add extensibility points

I've spent some time today figuring out why the BinaryPropertyListWriter was generating slightly different data from what I expected.

To figure that out, did some debugging tricks like:

Change the EqualityComparers which are passed to the BinaryPropertyListWriter
Dumping all IDs as they are being assigned in AssignID
Adding some diagnostic code to BinaryPropertyListParser to get the index, offset and summary of all values in the binary property list,
...

In the end, my life would be easier if:

I could just inherit from BinaryPropertyListParser and BinaryPropertyListWriter and override methods like DoParse.
I could pass a custom EqualityComparer to the BinaryPropertyListWriter when it's being created.

This PR makes some of the methods protected and adds a new constructor, just enough so that I can actually do that.

Hopefully this is OK with you.

claunia / plist-cil