Generalizes to support multiple migrators

There's now a `Migrator` class that additional migrators can extend.

Most migrators will want to extend `MigratorBase`, which handles the
tree traversal. For now, `MigratorBase` only operates on a single
stylesheet, with the dependency migration implemented only by
`ModuleMigrator`.

bin/sass_module_migrator.dart → bin/sass_migrator.dart

@@ -8,32 +8,45 @@ import 'dart:io';
 
 import 'package:args/args.dart';
 
-import 'package:sass_module_migrator/src/migrator.dart';
+import 'package:sass_migrator/src/migrator.dart';
+import 'package:sass_migrator/src/migrators/module.dart';
+
+final migrators = <Migrator>[ModuleMigrator()];
 
 void main(List<String> args) {
-  var argParser = new ArgParser()
+  var argParser = ArgParser(usageLineLength: 80)
     ..addFlag('dry-run',
         abbr: 'n',
         help: 'Show which files would be migrated but make no changes.')
     ..addFlag('verbose',
         abbr: 'v',
         help: 'Print text of migrated files when running with --dry-run.')
-    ..addFlag('help', abbr: 'h', help: 'Print help text.', negatable: false)
-    ..addFlag('recursive',
-        abbr: 'r',
-        help:
-            'Migrate all dependencies in addition to the entrypoints themselves.');
+    ..addFlag('help', abbr: 'h', help: 'Print help text.', negatable: false);
+
+  for (var migrator in migrators) {
+    argParser.addSeparator('${migrator.name} migrator\n' +
+        '=' * 80 +
+        '\n${migrator.description}\n${migrator.argParser.usage}');
+    argParser.addCommand(migrator.name, migrator.argParser);
+  }
   var argResults = argParser.parse(args);
 
-  if (argResults['help'] == true || argResults.rest.isEmpty) {
-    print('Migrates one or more Sass files to the new module system.\n\n'
-        'Usage: sass_migrate_to_modules [options] <entrypoint.scss ...>\n\n'
-        '${argParser.usage}');
-    exitCode = 64;
+  if (argResults['help'] == true || argResults.command == null) {
+    _printUsage(argParser);
     return;
   }
 
-  var migrated = migrateFiles(argResults.rest);
+  Migrator migrator;
+
+  try {
+    migrator = migrators
+        .singleWhere((migrator) => migrator.name == argResults.command.name);
+  } on StateError {
+    _printUsage(argParser);
+    return;
+  }
+  migrator.argResults = argResults.command;
+  var migrated = migrator.migrateFiles(argResults.command.rest);
 
   if (migrated.isEmpty) {
     print('Nothing to migrate!');
@@ -56,3 +69,10 @@ void main(List<String> args) {
     }
   }
 }
+
+void _printUsage(ArgParser argParser) {
+  print('Runs a migrator on one or more Sass files.\n\n'
+      'Usage: sass_migrator <migrator> [options] <entrypoint.scss ...>\n\n'
+      '${argParser.usage}');
+  exitCode = 64;
+}