replace links that have .md suffixes with clean links

to aid viewing the raw markdown source in e.g. a gitlab source browser,
or to aid navigation in vim with "gf" style commands to jump between
files, allow the markdown source to specify foo.md or whatever/index.md
explicitly, yet generate the clean URLs for linking in the HTML output

this assumes that nginx is serving "foo" with foo.html, and "bar/" with
bar/index.html

Signed-off-by: Brian S. Stephan <bss@incorporeal.org>

tests/instance/pages/file-with-index.md-link.md

@@ -0,0 +1,6 @@
+[Cool](cool/index.md)
+[Anchored Cool](cool/index.md#anchor)
+[This Index](index.md)
+[Anchored This Index](index.md#anchor)
+[Parent](../index.md)
+[Anchored Parent](../index.md#anchor)

tests/instance/pages/file-with-md-link.md

@@ -0,0 +1,4 @@
+[Foo](foo.md)
+[Anchored Foo](foo.md#anchor)
+[Sub Foo](sub/foo.md)
+[Anchored Sub Foo](sub/foo.md#anchor)