Adds two filters and composer.json

Hi Konstantin,

I hope you're well. This plugin has saved me a ton of work tonight, so I thank you for some sleep. :)

I've added a filter called `engg_image_url` to the remote image URL, and another to the post content before it's saved, called `engg_post_content`. I've also added a `composer.json` file, so this repo can be included via Composer, if a developer so desires.

Hope the changes are useful, and you find them suitable to merge!

Cheers,

Simon