Support for custom XML attributes

[sandrokeil]

Thank you for this library. 👍

Are there any reasons why custom attributes are not supported like described in 4.1 Adding XML Attributes to GraphML Elements?

Only complex types are supported, but custom XML attributes would be useful too.

[clue]

Hi @sandrokeil, can you be more specific in what you're missing and what your motivation is?

Right now, this library does import (simple) GraphML-attributes just fine, but it does not export them yet (#5). What what be your use case for XML attributes and complex types?

[sandrokeil]

@clue I need some custom attributes for the node XML nodes. See my PR test case for more details.

[sandrokeil]

ping @clue ;-)