Add Flags + Pattern Constructor to Regex

This allows creating patterns with a custom set of flags, whitout
having to manage the setate of an `Options` instance. This is useful
because `Options` is `IDisposable` and needs a bit of taking care of.

Fixes #11

src/IronRure/Regex.cs

@@ -33,6 +33,13 @@ public Regex(string pattern, Options opts)
             : this(pattern, opts.Raw, (uint)DefaultFlags)
         {}
 
+        /// <summary>
+        ///   Create a new regex instance from the given pattern and flags.
+        /// </summary>
+        public Regex(string pattern, RureFlags flags)
+            : this(pattern, IntPtr.Zero, (uint)flags)
+        {}
+
         /// <summary>
         ///   Create a new regex instance from the given pattern, with the given
         ///   options applied and with the given flags enabled.

test/IronRureTests/RegexTests.cs

@@ -42,6 +42,15 @@ public void Regex_CreateWithFlags_Succeeds()
             }
         }
 
+        [Fact]
+        public void Regex_CreateWithFlagsOnly_Succeeds()
+        {
+            using (var reg = new Regex(@"Δ", RureFlags.Unicode | RureFlags.Casei))
+            {
+                Assert.True(reg.IsMatch("δ"));
+            }
+        }
+
         [Fact]
         public void Regex_CreateWithInvalidPattern_ThrowsException()
         {