also output the measurement table with markdown (#379)

Previously when providing `--markdown` flag, only the tests table would be
formatted in Markdown format.

With this patch, both the tests and the measurements table are formatted using
markdown.

Follow-up to #372.

interop.py

@@ -215,6 +215,8 @@ def get_letters(result):
 
         if len(self._measurements) > 0:
             t = prettytable.PrettyTable()
+            if self._markdown:
+                t.set_style(prettytable.MARKDOWN)
             t.hrules = prettytable.ALL
             t.vrules = prettytable.ALL
             rows = {}