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;

static/octicon-mixins.less

@@ -1,15 +1,12 @@
-.icon() {
+.icon(@size, @display: inline-block) {
   font-family: 'Octicons Regular';
   font-weight: normal;
   font-style: normal;
-  display: inline-block;
+  display: @display;
   line-height: 1;
   -webkit-font-smoothing: antialiased;
   text-decoration: none;
-}
 
-.icon(@size) {
-  .icon;
   font-size: @size;
   width: @size;
   height: @size;