config: return nil on unknown option instead of raising

Returning `nil` on unknown option was default behaviour for Pry v0.12.x. In
e5556a2 I changed that but I am not sure if it
was intentional. This breaks plugins such as
pry-theme (kyrylo/pry-theme#59).

Returning `nil` makes more sense, because we can write code like this:

```
Pry.config.foo ||= 123
```

...whereas as of now this is no longer possible and you would need to use
`respond_to?` to achieve the same effect.

CHANGELOG.md

@@ -5,6 +5,12 @@
 * Fixed bug where on invalid input only the last syntax error is displayed
   (instead of all of them) ([#2117](https://github.com/pry/pry/pull/2117))
 
+#### API changes
+
+* `Pry::Config` returns `nil` on undefined option instead of raising
+  `NoMethodError` (usually invoked via `Pry.config.foo_option` calls)
+  ([#1877](https://github.com/pry/pry/pull/1877))
+
 ### [v0.13.0][v0.13.0] (March 21, 2020)
 
 #### Features

lib/pry/config.rb

@@ -247,7 +247,8 @@ def method_missing(method_name, *args, &block)
       elsif @custom_attrs.key?(name)
         self[name]
       else
-        super
+        nil
+        # super
       end
     end
 

spec/config_spec.rb

@@ -150,8 +150,8 @@
     end
 
     context "when invoked method is not an option" do
-      it "raises NoMethodError" do
-        expect { subject.foo }.to raise_error(NoMethodError)
+      it "returns nil" do
+        expect(subject.foo).to be_nil
       end
     end