Simplify/clarify logic in writing-a-plugin docs

Continuing the discussion from https://github.com/gulpjs/gulp/issues/325 to discuss the merits of using else-if statements to simplify and clarify intent, as well as DRY it up.

This PR aims to:
- Simplify/DRY-up code where multiple `this.push(file)` and `return callback();` statements were before.
- Clarify intent, running `callback();` without a return statement; though, still not sure if that was even the original intent.