Use display: inline for editor fold marker

This prevents it from wrapping to the next line when the window
is narrower than the line length.

Closes #211

static/editor.less

@@ -53,7 +53,7 @@
 }
 
 .editor .fold-marker:after {
-  .icon(0.8em);
+  .icon(0.8em, inline);
   content: @ellipsis;
   padding-left: 0.2em;
   position: relative;