Add follow_url_fallback config option (#172)

* Add follow_link_fallback config option
* Update README.md
* Rename follow_link_fallback to follow_url_fallback. Change logic of configuring

doc/telekasten.txt

@@ -418,6 +418,13 @@ telekasten.setup({opts})
 
         Default: `telescope-media-files`
 
+                                    *telekasten.settings.follow_url_fallback*
+    follow_url_fallback:~
+        A customizable fallback handler for urls.
+        When it set to nil, Telekasten will use `xdg-open` or `open`
+
+        Default: `nil`
+        Example: `"call jobstart('firefox --new-window {{url}}')"`
                                                *telekasten.calendar_opts*
     -----------------------------------
     Valid keys for {opts.calendar_opts}