New option to log only function names

I'd like to add a new flag to xdebug_start_code_coverage to log function names only (and not every single line).

This new flag makes xdebug_start_code_coverage log function names, and how many times they were called.

You can try my code by building it and then running it on your sample code for the code coverage (from http://xdebug.org/docs/code_coverage), and adding the new flag XDEBUG_CC_FUNC_ONLY to it:

```php
xdebug_start_code_coverage(XDEBUG_CC_FUNC_ONLY);

function a($a) {
    echo $a * 2.5;
}

function b($count) {
    for ($i = 0; $i < $count; $i++) {
        a($i + 0.17);
    }
}

b(6);
b(10);

var_dump(xdebug_get_code_coverage());
```

Thanks!