Fix escapes

If you want to include a literal ${macro call}, you can preface it with
a backslash like \${macro call}.

Signed-off-by: Alek Ratzloff <alekratz@gmail.com>

markup.py

@@ -96,10 +96,14 @@ class MarkupParser:
 
     def parse(self):
         while self.c:
-            if self.c in r"\$":
+            if self.is_match("${"):
                 mac = self.parse_macro()
                 self.parts += [mac]
             else:
+                # skip past the escape
+                if self.is_match(r"\${"):
+                    self.adv(3)
+                    self.parts += [MarkupRaw("${")]
                 raw = self.parse_raw()
                 self.parts += [raw]