Add possibility to get a node attribute

[ppcamp]

First of all, congrats to you! Your package is the only one that i've found that can handle with dumbs htmls inputs (broken ones).

Can you make the attributes of a Node being public? I mean, for now I'm using some regex approachs to got some attrs (like name/id/value).

// Current approach:
tree.getElementsByName('javax.faces.ViewState').first;
final jsfaces = Re.firstGroup(el.toString(), r'value=\s*"(.*)"');

// An interesting approach:
final jsfaces = tree.getElementsByName('javax.faces.ViewState').first.attributes['value'];