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 https://github.com/bowman2001/hugo/tree/plainify-strings-for-meta and comment on the pull request. Your help would be much appreciated!

Thanks, Georg