Add support for TimeSpan options

Fixes #173

Author: 0xced

Diff for: src/CommandLine/Core/TypeConverter.cs

@@ -51,9 +51,17 @@ private static Maybe<object> ChangeTypeScalar(string value, Type conversionType,
 
         private static object ConvertString(string value, Type type, CultureInfo conversionCulture)
         {
-            return Convert.ChangeType(value, type, conversionCulture);
+            try
+            {
+                return Convert.ChangeType(value, type, conversionCulture);
+            }
+            catch (InvalidCastException)
+            {
+                // Required for converting from string to TimeSpan because Convert.ChangeType can't
+                return System.ComponentModel.TypeDescriptor.GetConverter(type).ConvertFrom(null, conversionCulture, value);
+            }
         }
-        
+
         private static Result<object, Exception> ChangeTypeScalarImpl(string value, Type conversionType, CultureInfo conversionCulture, bool ignoreValueCase)
         {
             Func<object> changeType = () =>

Diff for: tests/CommandLine.Tests/CommandLine.Tests.csproj

@@ -95,6 +95,7 @@
     <Compile Include="Fakes\Options_With_Two_Options_Having_Required_Set_To_True.cs" />
     <Compile Include="Fakes\Options_With_Required_Set_To_True.cs" />
     <Compile Include="Fakes\Options_With_Required_Set_To_True_Within_Same_Set.cs" />
+    <Compile Include="Fakes\Options_With_TimeSpan.cs" />
     <Compile Include="Fakes\Help_Fakes.cs" />
     <Compile Include="Fakes\IInterface_With_Two_Scalar_Options.cs" />
     <Compile Include="Fakes\Immutable_Verb_Fakes.cs" />

Diff for: tests/CommandLine.Tests/Fakes/Options_With_TimeSpan.cs

@@ -0,0 +1,12 @@
+// Copyright 2005-2015 Giacomo Stelluti Scala & Contributors. All rights reserved. See License.md in the project root for license information.
+
+using System;
+
+namespace CommandLine.Tests.Fakes
+{
+    public class Options_With_TimeSpan
+    {
+        [Option('d', "duration")]
+        public TimeSpan Duration { get; set; }
+    }
+}

Diff for: tests/CommandLine.Tests/Unit/Core/InstanceBuilderTests.cs

@@ -1023,6 +1023,22 @@ public void Parse_Guid(string[] arguments, Options_With_Guid expected)
             // Teardown
         }
 
+        [Fact]
+        public void Parse_TimeSpan()
+        {
+            // Fixture setup
+            var expectedResult = new Options_With_TimeSpan { Duration = TimeSpan.FromMinutes(42) };
+
+            // Exercize system 
+            var result = InvokeBuild<Options_With_TimeSpan>(
+                new[] { "--duration=00:42:00" });
+
+            // Verify outcome
+            expectedResult.ShouldBeEquivalentTo(((Parsed<Options_With_TimeSpan>)result).Value);
+
+            // Teardown
+        }
+
         public static IEnumerable<object> RequiredValueStringData
         {
             get