Make image quality configurable for posts single headers

[fmueller]

I use some high-quality photos for blog posts and found the quality of 100 for downsized images a bit too much. I don't see a difference with 90 quality and save a lot of bandwidth.

This PR makes this configurable for 'hero' and 'full' themes. Default is 90.

If this does not match with your design goals for this template, please close this or use it as an inspiration.

Thank you for making this open source. :)

[uPagge]

It's a great idea to make this parameter configurable.

[fmueller]

Uh damn it, sorry all the other commits from my custom fork slipped in here. This was unintended. I don't have the permissions to clean it up.

Please feel free to close this and do the changes structured.

Sorry for the inconvenience.