Add parameters to output split png files

[heringerp]

Adds a parameter to output the result as png and another one to split each subfigure of the diagram into a different file. Implements #10

[danydoerr]

@heringerp Thanks, Peter for this implementation, very helpful. I only have the following kind requests:

• Please make the file format option more generic, e.g. '--format ' with fixed choices corresponding to all formats supported by matplotlib. Default should be pdf.
• Please add an option to change the file prefix of the stored files.

Thanks!

[subwaystation]

:fire:

[danydoerr]

Thanks @heringerp, fantastic work!