Add method with combined SelectAttr and ExistsAttr functionality

[moredure]

Attr method should allow to use returned value in case attr exists (even for attributes without value) or discard value in case it's missing.

[mathieu-aubin]

Can you elaborate on what this does? Is it getting used at all within the project? Sorry i had to ask. Comments add relevant info that allows people like me, which dont have involvement or barely any into a project understand and maybe, just maybe, give a f*** about what others do in general.

[zhengchun]

Sorry, I read all the questions or PR, but some of question were not answered in time and then forgot for a long time.

I had merged PR #44 that add new mthod ExistsAttr(), new method Attr) (val string, exists bool) is not necessary, use SelectAttr() if you want to get the attribute value, or use ExistsAttr() if just check the specified node whether exists the attribute.

[moredure]

@zhengchun, thank you!