Add autocomplete for text prompt

Closes #166

src/Spectre.Console/Internal/TypeConverterHelper.cs

@@ -0,0 +1,55 @@
+using System;
+using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
+using System.Reflection;
+
+namespace Spectre.Console.Internal
+{
+    internal static class TypeConverterHelper
+    {
+        public static string ConvertToString<T>(T input)
+        {
+            return GetTypeConverter<T>().ConvertToInvariantString(input);
+        }
+
+        [SuppressMessage("Design", "CA1031:Do not catch general exception types")]
+        public static bool TryConvertFromString<T>(string input, [MaybeNull] out T result)
+        {
+            try
+            {
+                result = (T)GetTypeConverter<T>().ConvertFromInvariantString(input);
+                return true;
+            }
+            catch
+            {
+                result = default;
+                return false;
+            }
+        }
+
+        public static TypeConverter GetTypeConverter<T>()
+        {
+            var converter = TypeDescriptor.GetConverter(typeof(T));
+            if (converter != null)
+            {
+                return converter;
+            }
+
+            var attribute = typeof(T).GetCustomAttribute<TypeConverterAttribute>();
+            if (attribute != null)
+            {
+                var type = Type.GetType(attribute.ConverterTypeName, false, false);
+                if (type != null)
+                {
+                    converter = Activator.CreateInstance(type) as TypeConverter;
+                    if (converter != null)
+                    {
+                        return converter;
+                    }
+                }
+            }
+
+            throw new InvalidOperationException("Could not find type converter");
+        }
+    }
+}