Default argument values aren't language agnostic

Using an example to demonstrate the issue:

If `paltrace` is called while stopped in some swift code, the user would expect to be able to run `paltrace someValidSwiftExpression`. As such, `paltrace` should interpret input based on the active language at a breakpoint. However, the `paltrace` command has a default view of `(id)[[UIApplication sharedApplication] keyWindow]`, but this expression would have to be evaluated as objc.

The solution that seems to strike the right balance to me is:

> Remove the ability for commands to declare defaults in `fb.FBCommandArgument`, which would force commands to knowingly handle a default.

Other possibilities are:
1. Allow commands a way to check if an argument was provided by a default
2. Use a heuristic to recognize when a value is swift or objc
3. Use a known "keyword" for the default

The second option would be nice to have in general, and could be desirable in addition to whatever solution chosen for handling defaults.