Add custom display strings

[pezipink]

This PR attempts to add two related features;

1. eye.SetDisplayString(type, displayString) where the display string is similar to DebuggerDisplayAttribute. That is, the string may contain {PropertyNames} which will be replaced with the .ToString() version of the property in question.

2. eye.SetFormatter f accepts a callback function that allows the user to generate display strings directly for each instance within the context of their program.

Notes:

• I just realised there is a regex helper, maybe my regex could should be changed to use it, though it is very simple.
• No tests yet. I have tested it pretty well and it is quite defensive.
• I should really update the docs!

Future enhancements

• Display strings only work with properties, would be cool to include members and methods as well.