Implement 'default values' in expressions.

[zanerock]

Overview

It's useful to be able to provide default values for non-defined parameters. How about:

• If a parameter ends in a 't', it defaults to true.
• If a parameter ends in an 'f', it defaults to false.
• If a parameter ends in an 'n' followed by a number, it defaults to that number.

Because we'd have to deal with quoting and such, we're not going to deal with default strings just yet.

Deliverables

• Implement t/f defaults.
• Implement numeric default support.
• Ensure that the default appears in the square brackets (or cap Y or N in 'y/n' for bools).
• Ensure that hitting enter accepts the default and sets the parameter to that value.