Add interwiki index location setting

peppermint.guiconfig.json

@@ -18,6 +18,7 @@
 	"anonedits": { "type": "checkbox", "description": "Whether users who aren't logged in are allowed to edit your wiki.", "default": false},
 	"maxpagesize": { "type": "number", "description": "The maximum page size in characters.", "default": 135000},
 	"parser": { "type": "text", "description": "The parser to use when rendering pages. Defaults to an extended version of parsedown (http://parsedown.org/)", "default": "parsedown" },
+	"interwiki_index_location": { "type": "url", "description": "The location to find the interwiki wiki definition file, which contains a list of wikis along with their names, prefixes, and root urls. May be a URL, or simply a file path - as it's passed to file_get_contents()." },
 	"clean_raw_html": { "type": "checkbox", "description": "Whether page sources should be cleaned of HTML before rendering. It is STRONGLY recommended that you keep this option turned on.", "default": true},
 	"enable_math_rendering": { "type": "checkbox", "description": "Whether to enable client side rendering of mathematical expressions with MathJax (https://www.mathjax.org/). Math expressions should be enclosed inside of dollar signs ($). Turn off if you don't use it.", "default": true},
 	"users": { "type": "usertable", "description": "An array of usernames and passwords - passwords should be hashed with password_hash() (the hash action can help here)", "default": {