Allow skipping anchor checking by regex

To avoid needing to turn off anchor checking across the entire
documentation allow skipping based on matching the anchor against a
regex.

Some sites/pages use JavaScript to perform anchor assignment in a
webpage, which would require rendering the page to determine whether
the anchor exists. Allow fine grain control of whether the anchor is
checked based on pattern matching, until such stage as the retrieved
URLs can be passed through an engine for deeper checking on the HTML
doctree.

tests/test_build_linkcheck.py

@@ -14,12 +14,25 @@ from util import with_app
 
 
 @with_app('linkcheck', testroot='linkcheck', freshenv=True)
-def test_all(app, status, warning):
+def test_defaults(app, status, warning):
     app.builder.build_all()
 
     assert (app.outdir / 'output.txt').exists()
     content = (app.outdir / 'output.txt').text()
 
-    # expect all ok
-    assert not content
+    print(content)
+    # looking for #top should fail
+    assert "Anchor 'top' not found" in content
+    assert len(content.splitlines()) == 1
 
+
+@with_app('linkcheck', testroot='linkcheck', freshenv=True,
+          confoverrides={'linkcheck_anchors_ignore': ["^!", "^top$"]})
+def test_anchors_ignored(app, status, warning):
+    app.builder.build_all()
+
+    assert (app.outdir / 'output.txt').exists()
+    content = (app.outdir / 'output.txt').text()
+
+    # expect all ok when excluding #top
+    assert not content