Extended Static-Images Endpoint (#619)

* extended staticmap capabilities

* added allowRemoteMarkerIcons configuration option and restricted fetching of remote marker icons only when option is set to true;
asynchronously load all available icons in a settings object on server startup;
replaced fs.existsSync() call in serve_rendered when drawing marker icons with a check against available icons settings object;

* updated documentation for linecap parameter in staticmaps endpoint;
added linejoin parameter to staticmaps endpoint;

* added capability of staticmap endpoint to draw multiple paths

docs/config.rst

@@ -14,6 +14,7 @@ Example:
         "root": "",
         "fonts": "fonts",
         "sprites": "sprites",
+        "icons": "icons",
         "styles": "styles",
         "mbtiles": ""
       },
@@ -31,6 +32,7 @@ Example:
       "serveAllFonts": false,
       "serveAllStyles": false,
       "serveStaticMaps": true,
+      "allowRemoteMarkerIcons": true,
       "tileMargin": 0
     },
     "styles": {
@@ -141,6 +143,13 @@ Optional string to be rendered into the raster tiles (and static maps) as waterm
 Can be used for hard-coding attributions etc. (can also be specified per-style).
 Not used by default.
 
+``allowRemoteMarkerIcons``
+--------------
+
+Allows the rendering of marker icons fetched via http(s) hyperlinks.
+For security reasons only allow this if you can control the origins from where the markers are fetched!
+Default is to disallow fetching of icons from remote sources.
+
 ``styles``
 ==========