Plotscripts as Plugin

There are three kinds of plots:
- Can plot one optimizer run on one benchmark
- Can compare multiple runs on one benchmark
- Can compare multiple runs of one optimizer with multiple runs of another optimizer on one benchmark

To manage all possible plotfunctions, implement them as Plugins and `HPOlib-plot` can easily select and create possible plots.

Start by locating the existing plot functions and the HPOlib-plot command mentioned in the issue. Determine how the three plot cases should be represented as plugins and how plots would be selected and created; done means all stated plot combinations are supported through that plugin mechanism.

Written by the indexing model from the issue text.