Chaining appends onto an element

The method `<<` returns `self` instead of the nodes array so multiple appends can be chained together.

For example:

    require 'ox'
    include Ox
    
    doc = Document.new
    
    elem1 = Element.new('elem1')
    elem2 = Element.new('elem2')
    
    p doc << elem1 << elem2

    # Currently returns this:
    # => [#<Ox::Element:0x007fe4b205a840 @value="elem1", @attributes=nil, @nodes=nil>, 
    #     #<Ox::Element:0x007fe4b205a750 @value="elem2", @attributes=nil, @nodes=nil>]

    # But should return this: 
    # => #<Ox::Document:0x007fad5b05a9a0 @value="", @attributes={}, @nodes=[
    #       #<Ox::Element:0x007fad5b05a888 @value="elem1", @attributes=nil, @nodes=nil>, 
    #       #<Ox::Element:0x007fad5b05a838 @value="elem2", @attributes=nil, @nodes=nil>]>