option to control supported attributes in markdown links

Signed-off-by: Andy Miller <rhuk@mac.com>

CHANGELOG.md

@@ -1,6 +1,8 @@
 # v1.6.26
 ## mm/dd/2020
 
+1. [](#improved)
+    * Added new configuration option to control the supported attributes in markdown links [#2882](https://github.com/getgrav/grav/issues/2882)
 1. [](#bugfix)
     * Fixed blueprint for `system.pages.hide_empty_folders` [#1925](https://github.com/getgrav/grav/issues/2925)
 

system/blueprints/config/system.yaml

@@ -515,6 +515,16 @@ form:
                             validate:
                                 type: bool
 
+                        pages.markdown.valid_link_attributes:
+                          type: selectize
+                          size: large
+                          label: PLUGIN_ADMIN.VALID_LINK_ATTRIBUTES
+                          help: PLUGIN_ADMIN.VALID_LINK_ATTRIBUTES_HELP
+                          placeholder: "rel, target, id, class, classes"
+                          classes: fancy
+                          validate:
+                            type: commalist
+
                 caching:
                     type: tab
                     title: PLUGIN_ADMIN.CACHING

system/config/system.yaml

@@ -59,6 +59,12 @@ pages:
     special_chars:                               # List of special characters to automatically convert to entities
       '>': 'gt'
       '<': 'lt'
+    valid_link_attributes:                       # Valid attributes to pass through via markdown links
+      - rel
+      - target
+      - id
+      - class
+      - classes
   types: [html,htm,xml,txt,json,rss,atom]        # list of valid page types
   append_url_extension: ''                       # Append page's extension in Page urls (e.g. '.html' results in /path/page.html)
   expires: 604800                                # Page expires time in seconds (604800 seconds = 7 days)

system/src/Grav/Common/Page/Markdown/Excerpts.php

@@ -87,7 +87,7 @@ class Excerpts
             );
 
             // Valid attributes supported.
-            $valid_attributes = ['rel', 'target', 'id', 'class', 'classes'];
+            $valid_attributes = Grav::instance()['config']->get('system.pages.markdown.valid_link_attributes');
 
             // Unless told to not process, go through actions.
             if (array_key_exists('noprocess', $actions)) {
@@ -232,6 +232,7 @@ class Excerpts
         $url_parts = is_string($url) ? $this->parseUrl($url) : $url;
         $actions = [];
 
+
         // if there is a query, then parse it and build action calls
         if (isset($url_parts['query'])) {
             $actions = array_reduce(