Add a setting for using locally installed Graphviz for generating diagrams.

The Google Charts service is only capable of rendering a limited range
of characters, so if you name your agents in an exotic language
generated diagrams are virtually useless being unable to make out.

With this change, Huginn embeds a locally generated SVG diagram instead
of relying on a third-party API if config.use_graphviz_dot is set to a
command name/path of dot(1).

For example, Google Charts API does not support kanji characters in its Graphviz generator.