feature(config): adding config.ignore to replace metadata (e.g. frontmatter) before linting

lib/markdownlint.js

@@ -53,8 +53,24 @@ function uniqueFilterForSorted(value, index, array) {
   return (index === 0) || (value > array[index - 1]);
 }
 
+function ignoreContent(content, config) {
+  if (typeof config.ignore === "function") {
+    // allow custom ignore callback
+    return config.ignore(content);
+  }
+
+  return content.replace(config.ignore, function removeNonLineBreaks(m) {
+    // maintain line breaks
+    return m.replace(/[^\n]+/g, "");
+  });
+}
+
 // Lints a single string
 function lintContent(content, config) {
+  // ignore portions of content
+  if (config.ignore) {
+    content = ignoreContent(content, config);
+  }
   // Parse content into tokens and lines
   var tokens = md.parse(content, {});
   var lines = content.split(shared.newLineRe);
@@ -170,6 +186,18 @@ function markdownlint(options, callback) {
   var config = options.config || { "default": true };
   var synchronous = (callback === markdownlintSynchronousCallback);
   var results = new Results();
+  if (config.ignore) {
+    if (config.ignore === "frontmatter") {
+      // see http://jekyllrb.com/docs/frontmatter/
+      config.ignore = /^---([\s\S]+?)---/;
+    }
+    if (typeof config.ignore !== "function" &&
+      !config.ignore.test &&
+      !config.ignore.exec
+    ) {
+      throw new TypeError("config.ignore must be a RegExp or Function");
+    }
+  }
   // Helper to lint the next file in the array
   function lintFilesArray() {
     var file = files.shift();