Blockdiag

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* Add node attribute: textcolor
* Add group attribute: textcolor
* Add edge attribute: textcolor
* Add diagram attributes: default_text_attribute
* Fix beginpoint shape and endpoint shape were reversed
* Fix bugs

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* Add diagram attributes: default_node_color, default_group_color and default_line_color
* Fix bugs

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* Add icon attribute to node
* Make transparency to background of PNG images
* Fix bugs

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* Fix bugs

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* Fix bugs

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* Fix bugs