Use yargs to parse the arguments passed to the linter

  This makes it possible to manually lint multiple files at once
without needing to run the linter multiple times.

  Also probably fixes the issue where an error in a single file would
cause all subsequent files to be printed at the end¹.

# References:
¹ https://travis-ci.org/mdn/browser-compat-data/builds/383532922#L7191

package.json

@@ -26,7 +26,8 @@
   "homepage": "https://github.com/mdn/browser-compat-data#readme",
   "devDependencies": {
     "ajv": "^5.0.1",
-    "mdn-confluence": "0.0.3"
+    "mdn-confluence": "0.0.3",
+    "yargs": "^11.1.0"
   },
   "scripts": {
     "confluence": "node ./node_modules/mdn-confluence/main/generate.es6.js --output-dir=.",

test/lint.js

@@ -1,19 +1,39 @@
-var fs = require('fs');
-var path = require('path');
-var {testStyle} = require('./test-style');
-var {testSchema} = require('./test-schema');
-var {testVersions} = require('./test-versions');
-var hasErrors, hasStyleErrors, hasSchemaErrors, hasVersionErrors = false;
-var filesWithErrors = {};
+const fs = require('fs');
+const path = require('path');
+const yargs = require('yargs');
+const {testStyle} = require('./test-style');
+const {testSchema} = require('./test-schema');
+const {testVersions} = require('./test-versions');
+
+const argv = yargs.alias('version','v')
+  .usage('$0 [[--] files...]', false, (yargs) => {
+    yargs.positional('files...', {
+      description: 'The files to lint',
+      type: 'string'
+    })
+  })
+  .help().alias('help','h').alias('help','?')
+  .parse(process.argv.slice(2));
+
+let hasErrors = false;
+let filesWithErrors = {};
 
 function load(...files) {
+  if (files.length === 1 && files[0] instanceof Array) {
+    files = files[0];
+  }
   for (let file of files) {
     if (file.indexOf(__dirname) !== 0) {
       file = path.resolve(__dirname, '..', file);
     }
 
+    if (!fs.existsSync(file)) {
+      continue; // Ignore non-existent files
+    }
+
     if (fs.statSync(file).isFile()) {
       if (path.extname(file) === '.json') {
+        let hasStyleErrors, hasSchemaErrors, hasVersionErrors = false;
         console.log(file.replace(path.resolve(__dirname, '..') + path.sep, ''));
         if (file.indexOf('browsers' + path.sep) !== -1) {
           hasSchemaErrors = testSchema(file, './../schemas/browsers.schema.json');
@@ -40,8 +60,8 @@ function load(...files) {
   }
 }
 
-if (process.argv[2]) {
-  load(process.argv[2])
+if (argv.files) {
+  load(argv.files)
 } else {
   load(
     'api',
@@ -60,7 +80,7 @@ if (process.argv[2]) {
 
 if (hasErrors) {
   console.log("");
-  console.log(`Problems in ${Object.keys(filesWithErrors).length} files:`);
+  console.warn(`Problems in ${Object.keys(filesWithErrors).length} files:`);
   for (let file in filesWithErrors) {
     console.log(file);
     testSchema(filesWithErrors[file]);