MaskyS's sandbox: Masky's Gist Input Filter (gist_filter) 8.x Port

This is a continuation of capuleto's port (https://www.drupal.org/sandbox/fl3a/2819998). Some time ago I inspected the codebase and it was of really good quality. There were just a few files missing and had to delete one D7 file. Later on, I decided to test the module. That's when I noticed that, the user cannot use all the display methods at once, and if the user changed the display method, ALL Gists throughout the site would be rendered that way. This is really unflexible, and means that if you wanted a gist link in one article and an embedded gist in another, you couldn't do it. So I rewrote the process() function and removed the 'choose a display method part'.

After working upon the initial code, the module/filter now replaces three tags:

[gist:xx] is replaced and a fully embedded gist is displayed. (embed method was chosen as the '[gistembed:xx]' tag would have been too long, and the embed method is likely the most used one.)

[gistcode:xx] is replaced by just the contents(code) of the gist, can come up handy when you are explaining a file little by little.

[gistlink:xx] is replaced by the link to the gist. This way, the user can use all three display methods simultaneously on their website without any issues. I have attached a screenshot demonstrating this.