Support highlight.js 10.4.0, bump add-on to version 2.1.0

Add the new CSS themes introduced by highlight.js 10.4.0, in order to
upload an add-on embedding that version to ATN. The new highlight.js has
improved support for Git diffs, courtesy of @fbezdeka. Thanks!

Bump version number of the add-on for the new ATN submission.

Fixes: #97

manifest.json

@@ -7,7 +7,7 @@
     }
   },
   "name": "Colored Diffs",
-  "version": "2.0.1",
+  "version": "2.1.0",
   "description": "Color diffs in messages formatted by Git or other version control systems",
   "author": "Vadim Atlygin",
   "homepage_url": "https://github.com/Qeole/colorediffs",

options/fill-css-list.js

@@ -85,6 +85,8 @@ const styles = [
     { "file": "solarized-dark",            "name" : "Solarized Dark"            },
     { "file": "solarized-light",           "name" : "Solarized Light"           },
     { "file": "srcery",                    "name" : "Srcery"                    },
+    { "file": "stackoverflow-dark",        "name" : "StackOverflow Dark"        },
+    { "file": "stackoverflow-light",       "name" : "StackOverflow Light"       },
     { "file": "sunburst",                  "name" : "Sunburst"                  },
     { "file": "tomorrow",                  "name" : "Tomorrow"                  },
     { "file": "tomorrow-night-blue",       "name" : "Tomorrow Night Blue"       },