Add configured XmlEncoder to allow encoding without prolog

[valentiay]

This PR add ability to configure XmlEncoder. At the moment only configuring prolog is implemented, buy config is free for extention.

For example,

@XmlCodec("Foo")
final case class Foo(a: Int, b: String, c: Double)

val foo = Foo(1, "abc", 1.0)

XmlEncoder[Foo].encodeWithConfig(foo, XmlEncoder.defaultConfig.withoutProlog)

will result to

<Foo><a>1</a><b>abc</b><c>1.0</c></Foo>

instead of default

<?xml version='1.0' encoding='UTF-8'?>
<Foo><a>1</a><b>abc</b><c>1.0</c></Foo>