Allow the option to not return an error (#21)

Allow the option to not return an error (#21)

Add `String()` method for options (#19)

This allows us to better understand test failures when options mismatch.

Also update README with description of `cmp` and `stringer` options

Add Equal methods for testing (#17)

This allows code that generates options to be compared without depending on being able to examine or infer the config.

Ensure other tags work if options tag is missing

Allow using a pointer to determine if an option was set (#13)