Plainify .Title, .Description and .Summary for meta tags

Hi everybody,

After compiling and running mage check, I just submitted the following pull request:

I also checked Hugo’s rendering time on a project with around 150 pages and couldn’t find a noticeable difference.

The proposed changes could also fix two longstanding requests and simplify the maintenance of this small task. This is my first pull request, please have a good look.

The main developers of Hugo are stretched thin, so maybe someone else could check out my changes at and comment on the pull request. Your help would be much appreciated!

Thanks, Georg