Support creating pseudo registers via the API

[xusheng6]

Sometimes certain bits within a register have a special meaning that is not known to the debugger, and the user would want to have a direct visual on them. In such cases, it would be great if we can allow the user to define a pseudo or custom register by supplying the name and expression to compute its value.

This is relevant to https://github.com/Vector35/debugger/issues/230 and https://github.com/Vector35/debugger/issues/466