Per-element-type encoding

[StokesMIDE]

When exporting to XML, there are various cases in which it would be useful to specify how individual element values get converted to text, similar to the BinaryElement encoding added in PR #99. Specifically, there are cases in which certain IntegerElements would be more easily read (by humans) if displayed in hex.

There are a couple of things that could be done (not mutually exclusive):

• Add a parameter to functions like ebml2xml() that allows a dictionary of element names and encoding functions. Simple, but would take some effort to rework the command-line utilities.
• Add an 'encoding' attribute to the element definitions in the schema. A bit more work, but likely a better approach.