Update setup.cfg flake8 section

- Add the _n gettext prefix as a builtin, resolves "F821 undefined
  name"
- Include the main executable when running flake8 from the top directory
  without specifying files
- Ignore E402,F401 module import violations for specific files

setup.cfg

@@ -19,8 +19,26 @@ ignore-bad-ideas = *.mo
 ignore = *.rej
 
 [flake8]
-builtins = _
+filename =
+    *.py
+    ./linkchecker
+builtins =
+    _
+    _n
 max-line-length = 80
+per-file-ignores =
+# In several files imports intentionally cause:
+# E402: module level import not at top of file
+# F401: module imported but unused
+    linkchecker: E402
+    setup.py: E402
+    linkcheck/__init__.py: E402,F401
+    linkcheck/bookmarks/chrome.py: E402
+    linkcheck/checker/httpurl.py: E402
+    linkcheck/htmlutil/htmlsoup.py: E402
+    linkcheck/parser/__init__.py: E402
+    tests/__init__.py: F401
+    
 extend-ignore =
 # https://pep8.readthedocs.org/en/latest/intro.html#error-codes
 # these are ignored by default: