Feature: Add config.php file support

[glensc]

Adds support for using config files

// Config::create() will load config/config.default.php
// and then merge with config/config.php (if it exists).
$config = \Xhgui\Profiler\Config::create();
$profiler = new \Xhgui\Profiler\Profiler($config);

This is preparation to add support for import.php cli tool:

• docker: Cannot import .xhprof files - PHP Fatal error: require(): Failed opening required xhgui/external/../vendor/autoload.php' xhgui#478 (comment)