new features

[sergesheff]

Hi, i've added few new features which were very useful in my project and i hope will be useful for other developers who using your lib.
List of added features:

• Added custom function for Element type marshalling. So user will able to json.Marshal svg after modifications
• Added a new function which allow to search nodes by a CharData
• Added a pointer to a parent element. Super useful when need to add a new node to the collection
• Added golang modules