Add support for setting a global env var prefix

[hhromic]

This PR implements support for setting a global env var prefix (see linked issue).
While writing common reusable args structs for our Go programs today, I realized on the value and usefulness of this feature in real-world applications and decided to implement it for go-arg.

The implementation adds a new EnvPrefix field in Config and therefore is only accessible when using NewParser().
Because the implementation adds the prefix to spec.env, the prefix is reflected everywhere, including usage help text.

To write the unit test, I had to extend parseWithEnv so a Config can be passed instead of hard-coding Config{}.

And last but not least, an example in the README was using os.Args instead of os.Args[1:] in p.Parse(). This probably came from a very outdated version, but I fixed it as well now.

Resolves #204

[alexflint]

Thanks! One request though: don't add the joining underscore, instead let the user put it in themselves if they need it.

[hhromic]

Thanks! One request though: don't add the joining underscore, instead let the user put it in themselves if they need it.

Done! The underscore is not hard-coded anymore :)

[hhromic]

Rebased this on top of latest master :)