Lexer option

This PR give ability to test the CLI version of the parser, using your own lexer.

For example, if you use a lexer like the one describe in [documentation](https://github.com/nikic/PHP-Parser/blob/master/doc/component/Lexer.markdown#user-content-token-offset-lexer), actually you can't test it with the CLI version without patching it.

Goal is to add a new option `--lexer` to select a new class that may be resolved in `include_path` through the standard PHP-Parser Autoloader 

Take an example. With a lexer as defined below
```php
<?php

namespace Bartlett\PhpParser\Lexer;

/**
 *
 *
 * @author Nikita Popov
 * @author Christoph M. Becker
 * @link   https://gist.github.com/nikic/04fce01e69ae5b7b44f8
 * @link   https://github.com/nikic/PHP-Parser/issues/136
 */

class TokenOffsets extends \PhpParser\Lexer\Emulative
{
    public function getNextToken(&$value = null, &$startAttributes = null, &$endAttributes = null)
    {
        $token = parent::getNextToken($value, $startAttributes, $endAttributes);
        $startAttributes['startOffset'] = $endAttributes['endOffset'] = $this->pos;
        return $token;
    }

    public function getTokens()
    {
        return $this->tokens;
    }
}
```
That is resolvable in the include_path (via `stream_resolve_include_path` php function), here are how to dump nodes and show token offsets attributes provided by this lexer version.

    $ php php-parse.php --lexer=Bartlett\PhpParser\Lexer\TokenOffsets --var-dump  file.php