Add flag for generating tests for only exported functions/methods

[cweill]

Something like -exportedonly or -only=exported.

[cweill]

Actually, this can easily be solved with #2 using -only=^[A-Z].*, so it may not be worth making a new flag. Consider including that example in flag description

[cweill]

cbb246e

[cweill]

This really needs a flag.