Added option to preserve dot file

[Stillerman]

Diagram("Label", outformat="png", preserve_graphviz_file=True)

will preserve the dot file generated for later use.

I could also refactor this so that dot file is preserved when outformat="dot"

This is a non-breaking change because preserve_graphviz_file is False by default.

[mingrammer]

Does outformat="dot" generate only a dot file?

[Stillerman]

No, but I can arrange for that.

[Stillerman]

How about when preserve_dot_file is true, the dot file is kept, and if outformat=dot then the file is produced regardless of preserve_dot_file.

[mingrammer]

IMO, outformat=dot may make users confused a bit due to it has different purposes other than the other image format options. preserve_dot_file option would be enough.

[steschuser]

Any changes this gets merged soonish?