Fix for ImageMagick path, options for \tikzpicture and show how to correctly use preamble

[DavidPowell]

I was attempting to reproduce this control systems example within a Jupyter notebook, on a Windows machine. I ran into the following problems, all of which are addressed in this pull request:

• I need to override the location of ImageMagick executable (it is called magick on my system, whereas the default on many systems seems to be convert). A new option --imagemagick has been added to enable this.
• I need to be able to pass additional options to the \tikzpicture command. A new option --pictureoptions has been added to enable this.
• I created a multi-line preamble, but had trouble passing this to the tikz magic. The solution was to surround it in double quotes, so I added this to the documentation.

I'm happy to change any of the options if necessary, or to split this PR into 3 separate ones covering each of the issues.

[DavidPowell]

An example notebook showing the working example is available here.