feat: allow ".md" extension when creating new files

Just say you want to create a file "foo.md" but you already have one
called "food.md"; you can't just type "foo" and hit enter because that
will just open the existing file. If we weren't in the terminal, and we
could actually distinguish between enter and, say, control-enter, then
we could use that as a cue to create the file. But we are in the
terminal, so we instead allow you to type "foo.md". That won't match
"food.md" unless you happen to have that string inside it (unlikely), so
in this way you can add an extension, optionally, as a way of forcing a
unique match and thus creating a new file. Subtle, but useful.

autoload/corpus.vim

@@ -602,7 +602,10 @@ function! corpus#choose(selection) abort
     execute 'cd ' . fnameescape(l:directory)
     echomsg 'Changed to: ' . l:directory
     if !empty(l:file)
-      execute 'edit '. fnameescape(l:file . '.md')
+      if l:file !~? '\v\.md$'
+        let l:file=l:file . '.md'
+      endif
+      execute 'edit '. fnameescape(l:file)
     endif
   endif
 endfunction