Add a script that lists all contributors to a release (#1181)

* Add a script that lists all contributors to a release

We will keep a contributors database (simple JSON file) where
each entry is a contributor (either a committer, either an issue reporter,
either both) with some nicknames (GitHub, and Twitter).
The file will be used to craft credits on our release blog posts and to ping
them on Twitter.

* Add templates

* Missing docstring

* Clean-up

* Tweak

docs/contributors/snippet.jinja2

@@ -0,0 +1,13 @@
+<!-- Blog post -->
+
+## Community contributions
+
+We’d like to thank these amazing people for their contributions to this release: {% for name, details in contributors.items() -%}
+    [{{ name }}](https://github.com/{{ details.github }}){{ '' if loop.last else ', ' }}
+{%- endfor %}.
+
+<!-- Twitter -->
+
+We’d like to thank these amazing people for their contributions to HTTPie {{ release }}: {% for name, details in contributors.items() if details.twitter -%}
+    @{{ details.twitter }}{{ '' if loop.last else ', ' }}
+{%- endfor %} 🥧