Php-cs-fixer: sprintf to interpolation fixer

[ntzm]

As of PHP 7 string interpolation is more performant than sprintf calls: https://blog.blackfire.io/php-7-performance-improvements-encapsed-strings-optimization.html (thanks @keradus)

-echo sprintf('foo %s baz', $bar);
+echo "foo $bar baz";

-echo sprintf('foo %s baz', $this->bar());
+echo "foo {$this->bar()} baz";

Obviously it gets a bit more complicated when you have expressions in the sprintf, so it shouldn't be changed here:

echo sprintf('foo %s baz', 1 + 1);

[julienfalque]

Maybe variable detection (to know whether string interpolation can be applied) could be extracted from YodaStyleFixer.

[soullivaneuh]

We also may convert this:

echo 'Hello '.$name.'!';

By this:

```php
echo "Hello {$name}!";