Nwdiag

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* Fix bugs

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* Add packetdiag_sphinxhelper

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* Add packetdiag which supports generating packet-header diaagram
* [nwdiag] Add diagram attribute: external_connector
* Update to new package structure (blockdiag >= 1.1.2)
* Allow to comment syntax
* Fix bugs

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* Accept N/A rack-unit
* Add fontfamily attribute for switching fontface
* Fix bugs

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* [rackdiag] Support multiple racks rendering
* [rackdiag] Add rack attribute: unit-height, weight, ampere, ascending
* [rackdiag] Support putting multiple items to same rack-unit

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* Add rackdiag which supports genarating rack-structure diagram
* Add docutils extension
* Fix bugs