Allow usage of all three display methods at once

Problem/Motivation

The drupal 8 port of this module, as well as D7, currently allow only one display method for the gists to be used at once, and all gists throughout the site will be rendered this way. This is unflexible.

Proposed resolution

Remove 'choose a method' form and use three different tags for each display method, so all three methods/tags can be used at once.
The three new

[gist:xx] for embedding gists using embed method.
[gistcode:xx] will display only the code contents of you gists.
[gistlink:xx] will generate the gist link and display it.

Remaining tasks

D8 patch needs review