Writer: add Double(d,precision) for one-shot double output

As proposed in other patches, it is convenient to pass a user-defined
precision for the (programmatic) output of a single double value
to an OutputStream.

This patch adds an additional overload with an explicit precision
argument to the (Pretty)Writer class templates.

include/rapidjson/prettywriter.h

@@ -51,6 +51,11 @@ class PrettyWriter : public Writer<OutputStream, SourceEncoding, TargetEncoding,
 	PrettyWriter& Int64(int64_t i64)	{ PrettyPrefix(kNumberType); Base::WriteInt64(i64);		return *this; }
 	PrettyWriter& Uint64(uint64_t u64)	{ PrettyPrefix(kNumberType); Base::WriteUint64(u64);	return *this; }
 	PrettyWriter& Double(double d)		{ PrettyPrefix(kNumberType); Base::WriteDouble(d);		return *this; }
+	//! Overridden for fluent API, see \ref Writer::Double()
+	PrettyWriter& Double(double d, int precision) {
+		int oldPrecision = Base::GetDoublePrecision();
+		return SetDoublePrecision(precision).Double(d).SetDoublePrecision(oldPrecision);
+	}
 
 	PrettyWriter& String(const Ch* str, SizeType length, bool copy = false) {
 		(void)copy;

include/rapidjson/writer.h

@@ -75,6 +75,20 @@ class Writer {
 	*/
 	Writer& Double(double d)		{ Prefix(kNumberType); WriteDouble(d);		return *this; }
 
+	//! Writes the given \c double value to the stream (explicit precision)
+	/*!
+		The currently set double precision is ignored in favor of the explicitly
+		given precision for this value.
+		\see Double(), SetDoublePrecision(), GetDoublePrecision()
+		\param d The value to be written
+		\param precision The number of significant digits for this value
+		\return The Writer itself for fluent API.
+	*/
+	Writer& Double(double d, int precision) {
+		int oldPrecision = GetDoublePrecision();
+		return SetDoublePrecision(precision).Double(d).SetDoublePrecision(oldPrecision);
+	}
+
 	Writer& String(const Ch* str, SizeType length, bool copy = false) {
 		(void)copy;
 		Prefix(kStringType);