[Console] Progress helper enhancements

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | ~
| License       | MIT
| Doc PR        | ~

Two enhancements:

1. The progress bar clears the current line before writing the current progress. This can cause some flickering in the terminal. I've modified the write method to append whitespace to the line to be written, so it matches the previous line's length.
2. Added a `setCurrent` method to set the current progress. Rather than advancing by 1 or more steps, sometimes you want to just set the current state. For example if you are downloading a file, and a callback provides you with the current download status. A workaround for this could be to keep track of the previous event, calculate the difference, and advancing by the diff. But it's easier to just set the current progress.

Sidenotes:
* The [`overwrite`](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Console/Helper/ProgressHelper.php#L387) method copied documentation of the Output's [`write`](https://github.com/symfony/symfony/blob/master/src/Symfony/Component/Console/Output/Output.php#L139) method. The difference is, the overwrite method does not handle an array of messages. I've updated the documentation for this.
* The helper uses `strlen` to calculate line lengths. This could cause a problem when using multibyte strings. I'd change it to `mb_strlen`, but I'm not sure if the `mb_string` extension is required by Symfony.