Add --extend-select command line argument

Implement `--extend-select` command line argument following what was
done for `--extend-ignore` in !233. This option can be used to
selectively add individual codes without overriding the default list
entirely.

Addresses the remaining item of issue #1061.

src/flake8/main/options.py

@@ -105,6 +105,7 @@ def register_default_options(option_manager):
     - ``--max-doc-length``
     - ``--indent-size``
     - ``--select``
+    - ``--extend-select``
     - ``--disable-noqa``
     - ``--show-source``
     - ``--statistics``
@@ -272,6 +273,18 @@ def register_default_options(option_manager):
         " For example, ``--select=E4,E51,W234``. (Default: %(default)s)",
     )
 
+    add_option(
+        "--extend-select",
+        metavar="errors",
+        default="",
+        parse_from_config=True,
+        comma_separated_list=True,
+        help=(
+            "Comma-separated list of errors and warnings to add to the list "
+            "of selected ones. For example, ``--extend-select=E4,E51,W234``."
+        ),
+    )
+
     add_option(
         "--disable-noqa",
         default=False,

src/flake8/style_guide.py

@@ -162,7 +162,14 @@ class DecisionEngine:
         )
         self.enabled_extensions = tuple(options.enable_extensions)
         self.all_selected = tuple(
-            sorted(self.selected + self.enabled_extensions, reverse=True)
+            sorted(
+                itertools.chain(
+                    self.selected,
+                    options.extend_select,
+                    self.enabled_extensions,
+                ),
+                reverse=True,
+            )
         )
         self.ignored = tuple(
             sorted(