Configurable thumb size

* **ADD:** `set_thumbnail_size()` takes 2 parameters: `$width` & `$height` *(int)*
and updates `thumb_width` and `_height` properties
* **MOD:** `_create_thumbnail()` uses `thumb_width` and `_height` properties
* **MOD:** `__construct()` loads config to set default `thumb_width` and `_height`
* **ADD:** Config keys to set default thumb size