Escape the hover text (#650)

In C#, you can get hover text that contains angle brackets.
Angle brackets don't need to be escaped in markdown.
The problem is that mdpopups.md2html doesn't escape our angle brackets
and causes invalid HTML to be generated, so let's escape our hover text
before we toss it into md2html.

plugin/hover.py

@@ -139,9 +139,9 @@ def hover_content(self, point, response: 'Optional[Any]') -> str:
             value = ""
             language = None
             if isinstance(item, str):
-                value = item
+                value = escape(item)
             else:
-                value = item.get("value")
+                value = escape(item.get("value"))
                 language = item.get("language")
             if language:
                 formatted.append("```{}\n{}\n```\n".format(language, value))