Add support for using custom rules.

test/rules/blockquote.js

@@ -0,0 +1,22 @@
+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "blockquote" ],
+  "description": "Rule that reports an error for blockquotes",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    params.tokens.filter(function filterToken(token) {
+      return token.type === "blockquote_open";
+    }).forEach(function forToken(blockquote) {
+      var lines = blockquote.map[1] - blockquote.map[0];
+      onError({
+        "lineNumber": blockquote.lineNumber,
+        "detail": "Blockquote spans " + lines + " line(s).",
+        "context": blockquote.line.substr(0, 7),
+        "range": null
+      });
+    });
+  }
+};

test/rules/every-n-lines.js

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+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "every-n-lines" ],
+  "description": "Rule that reports an error every N lines",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    var n = params.config.n || 2;
+    params.lines.forEach(function forLine(line, lineIndex) {
+      var lineNumber = lineIndex + 1;
+      if ((lineNumber % n) === 0) {
+        onError({
+          "lineNumber": lineNumber,
+          "detail": "Line number " + lineNumber,
+          "context": null,
+          "range": null
+        });
+      }
+    });
+  }
+};

test/rules/letters-E-X.js

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+// @ts-check
+
+"use strict";
+
+module.exports = {
+  "names": [ "letters-E-X", "letter-E-letter-X", "contains-ex" ],
+  "description": "Rule that reports an error for lines with the letters 'EX'",
+  "tags": [ "test" ],
+  "function": function rule(params, onError) {
+    params.tokens.filter(function filterToken(token) {
+      return token.type === "inline";
+    }).forEach(function forToken(inline) {
+      inline.children.filter(function filterChild(child) {
+        return child.type === "text";
+      }).forEach(function forChild(text) {
+        var index = text.content.toLowerCase().indexOf("ex");
+        if (index !== -1) {
+          onError({
+            "lineNumber": text.lineNumber,
+            "detail": null,
+            "context": text.content.substr(index - 1, 4),
+            "range": null
+          });
+        }
+      });
+    });
+  }
+};

test/rules/package.json

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+{
+  "name": "markdownlint-rules-test",
+  "version": "0.0.1",
+  "description": "Package of markdownlint custom rules used for testing",
+  "main": "rules.js",
+  "author": "David Anson (https://dlaa.me/)",
+  "homepage": "https://github.com/DavidAnson/markdownlint",
+  "license": "MIT",
+  "keywords": [
+    "markdownlint-rules"
+  ],
+  "private": true
+}

test/rules/rules.js

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+// @ts-check
+
+"use strict";
+
+var blockquote = require("./blockquote");
+module.exports.blockquote = blockquote;
+
+var everyNLines = require("./every-n-lines");
+module.exports.everyNLines = everyNLines;
+
+var lettersEX = require("./letters-E-X");
+module.exports.lettersEX = lettersEX;
+
+module.exports.all = [
+  blockquote,
+  everyNLines,
+  lettersEX
+];