feat: add --help flag

This adds a `--help` flag which takes precedence over all other arguments or positionals.

Closes #249

markdownlint-cli2.js

@@ -912,19 +912,31 @@ const main = async (params) => {
   let fixDefault = false;
   // eslint-disable-next-line unicorn/no-useless-undefined
   let configPath = undefined;
+
+  let shouldShowHelp = false;
   const argvFiltered = (argv || []).filter((arg) => {
     if (configPath === null) {
       configPath = arg;
       return false;
-    } else if (arg === "--config") {
+    } 
+    if (arg === "--config") {
       configPath = null;
       return false;
-    } else if (arg === "--fix") {
+    }
+    if (arg === "--fix") {
       fixDefault = true;
       return false;
     }
+    if (arg === '--help') {
+      shouldShowHelp = true;
+      return false;
+    }
     return true;
   });
+  if (shouldShowHelp) {
+    showHelp(logMessage);
+    return 2;
+  }
   // Read argv configuration file (if relevant and present)
   let optionsArgv = null;
   let relativeDir = null;

test/markdownlint-cli2-test.js

@@ -611,3 +611,31 @@ test("custom fs, using fsMock simulating symbolic links", (t) => {
       t.is(exitCode, 1);
     });
 });
+
+test("--help", (t) => {
+  t.plan(2);
+  const stdouts = [];
+  return markdownlintCli2({
+    "argv": [ "--help" ],    
+    "logMessage": (msg) => stdouts.push(msg),
+    "logError": (msg) => t.fail(`message logged: ${msg}`)
+  })
+    .then((exitCode) => {
+      t.is(exitCode, 2);
+      t.regex(stdouts.join('\n'), /Syntax: markdownlint-cli2/u);
+    });
+});
+
+test("--help, using globs", (t) => {
+  t.plan(2);
+  const stdouts = [];
+  return markdownlintCli2({    
+    "argv": [ "README.md", "--help" ],    
+    "logMessage": (msg) => stdouts.push(msg),
+    "logError": (msg) => t.fail(`message logged: ${msg}`)
+  })
+    .then((exitCode) => {
+      t.is(exitCode, 2);
+      t.regex(stdouts.join('\n'), /Syntax: markdownlint-cli2/u);
+    });
+})