Add reading time

[pitscher]

Hey, although the PR #223 is in review I got another idea.
What about the possibility to add the reading time to e.g. posts.
Hugo can compute this pretty easily. I added this + a custom reading_time config for each language + the option for a user to specify for which content types such a reading time should be shown (to give them as much flexibility as possible).

Example
A user with a website that hosts blog posts and projects (all different types of content pages) can decide to only display a posts reading time (and not the reading time of a projects page).

It looks like this (translation support included):

Would you be happy to see another PR for this? 🤓

[pitscher]

By the way: I saw that someone requested this to be added (#12) - however, it seems like it was not done.
AFAIK the theme does not really support reading time. Hence my implementation.