Ignore "page size" param.

[mjpyama]

I used v4.1.2 cli with over 19,000 heavy posts in WordPress.
And I use command with "-l" param(page size), but always occurred memory error.
I checked the code "srdb.class.php", and found follows:

public $page_size = 50000;

But command param keyname is "pagesize". no underscore. so it is ignored.
I edited $page_size = 50000; to $page_size = 500;, then command successed.

[gianluigi-icit]

thank you for the issue, I fixed on PR #358