fix(unparser): format values with parser ParsingCulture for round-trip

FormatCommandLine wrote option and value arguments using the ambient
thread culture via value.ToString(), while the parser reads them back
with ParserSettings.ParsingCulture (invariant by default). On threads
whose culture uses a comma decimal separator (for example de-DE) a
double 1.5 was unparsed as "1,5" and reparsed as 15, and a sequence
[1.5, 2.5] became "1,5,2,5" and reparsed as four elements [1,5,2,5],
so the format/parse round-trip was broken.

Format IFormattable values (numeric and date/time types) with the
parser ParsingCulture instead of the current thread culture, for both
scalar and separator-joined sequence values. Other types keep their
default string representation, so behaviour on invariant/en-US is
unchanged.

Add round-trip tests under de-DE for a scalar double and a
comma-separated double sequence.

tests/CommandLine.Tests/Fakes/Options_With_Double_Sequence.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.Collections.Generic;
+
+namespace CommandLine.Tests.Fakes
+{
+    class Options_With_Double_Sequence
+    {
+        [Option("dbls", Separator = ',')]
+        public IEnumerable<double> DoubleSequence { get; set; }
+    }
+}