Closes a bit of #129.

Using a fully-fledged compile-error testsuite is a bit overkill, but we still want to make sure that the derive crate generates reasonable error messages. That's what this "example" is for. In the following, there are blocks of lines, one marked as POSITIVE and multiple lines marked as NEGATIVE. The committed version of this file should only contain POSITIVE. In order to run a test, comment out the POSITIVE line, and use a NEGATIVE line instead, and manually check whether you see a reasonable error message – ideally the error message indicated by the comment. One way to do this is:

$ cargo build --example test_compile_errors_manually

Example output with the type that I did in #129:

$ cargo build --example test_compile_errors_manually
   Compiling uutils-args v0.1.0 (/home/user/workspace/uutils-args)
error: proc-macro derive panicked
  --> examples/test_compile_errors_manually.rs:24:10
   |
24 | #[derive(Arguments)]
   |          ^^^^^^^^^
   |
   = help: message: expected '=' after '[' in flag pattern

It's not a particularly nice error message, but it's more readable than the current called `Option::unwrap()` on a `None` value.