graphviz layer

This contains a forked version of graphviz-dot-mode that enables a live-reload type work flow for editing `.dot` files. When live-preview is enabled, saving the file will automatically trigger a compilation and reload of the image buffer associated with the file.

To use this configuration layer, add it to your ~/.spacemacs. You will need to add graphviz to the existing dotspacemacs-configuration-layers list in this file.

If the live preview is not always updating the rendered image properly, you can try to adjust the value of graphviz-dot-revert-delay higher to give the compiler more time to finish generating the file before reverting the buffer.