Moved the Option class to its own file in anticipation of working on it.

pkg/args/lib/src/options.dart

+library options;
+
+/**
+ * A command-line option. Includes both flags and options which take a value.
+ */
+class Option {
+  final String name;
+  final String abbreviation;
+  final List<String> allowed;
+  final defaultValue;
+  final Function callback;
+  final String help;
+  final Map<String, String> allowedHelp;
+  final bool isFlag;
+  final bool negatable;
+  final bool allowMultiple;
+
+  Option(this.name, this.abbreviation, this.help, this.allowed,
+      this.allowedHelp, this.defaultValue, this.callback, {this.isFlag,
+      this.negatable, this.allowMultiple: false}) {
+
+    if (name.isEmpty) {
+      throw new ArgumentError('Name cannot be empty.');
+    } else if (name.startsWith('-')) {
+      throw new ArgumentError('Name $name cannot start with "-".');
+    }
+
+    // Ensure name does not contain any invalid characters.
+    if (_invalidChars.hasMatch(name)) {
+      throw new ArgumentError('Name "$name" contains invalid characters.');
+    }
+
+    if (abbreviation != null) {
+      if (abbreviation.length != 1) {
+        throw new ArgumentError('Abbreviation must be null or have length 1.');
+      } else if(abbreviation == '-') {
+        throw new ArgumentError('Abbreviation cannot be "-".');
+      }
+
+      if (_invalidChars.hasMatch(abbreviation)) {
+        throw new ArgumentError('Abbreviation is an invalid character.');
+      }
+    }
+  }
+
+  static final _invalidChars = new RegExp(r'''[ \t\r\n"'\\/]''');
+}