Runtime configuration

Related to #85

Everything should now be configured at runtime, with the exception of
the `Pleroma.HTML` scrubbers (the scrubbers used can be
changed at runtime, but their configuration is compile-time) because
it's building a module with a macro.

test/config_test.exs

@@ -4,6 +4,7 @@ defmodule Pleroma.ConfigTest do
   test "get/1 with an atom" do
     assert Pleroma.Config.get(:instance) == Application.get_env(:pleroma, :instance)
     assert Pleroma.Config.get(:azertyuiop) == nil
+    assert Pleroma.Config.get(:azertyuiop, true) == true
   end
 
   test "get/1 with a list of keys" do
@@ -20,6 +21,22 @@ defmodule Pleroma.ConfigTest do
              )
 
     assert Pleroma.Config.get([:azerty, :uiop]) == nil
+    assert Pleroma.Config.get([:azerty, :uiop], true) == true
+  end
+
+  test "get!/1" do
+    assert Pleroma.Config.get!(:instance) == Application.get_env(:pleroma, :instance)
+
+    assert Pleroma.Config.get!([:instance, :public]) ==
+             Keyword.get(Application.get_env(:pleroma, :instance), :public)
+
+    assert_raise(Pleroma.Config.Error, fn ->
+      Pleroma.Config.get!(:azertyuiop)
+    end)
+
+    assert_raise(Pleroma.Config.Error, fn ->
+      Pleroma.Config.get!([:azerty, :uiop])
+    end)
   end
 
   test "put/2 with a key" do