feat: add support for marker on higher/lower scores

[sebastienlange]

With this new feature, it plots a 'o' marker each time a higher/lower score is reached. When early stopping, it's particularly usefull to plot the marker on the metric you're looking. Markers show when I saved my best models.

[stared]

@sebastienlange Let's me share the big picture:

I want to create it in a way that it is possible to add custom plots. Not only cost function trends but also misclassified pictures, etc. (E.g. as in: https://github.com/stared/keras-interactively-piterpy2018/blob/master/talk.ipynb; see also https://github.com/waleedka/hiddenlayer for some ideas.)

Otherwise, at some point, it will get too clunky, and still not flexible enough. So, the ideas is to have a few default plots, plus a possibility to either change them, or create one's own.

One way to go is functional. We create subplots as functions that take ax parameters, and others (like logs, but can be also some external stuff).

To make it clear - still I want livelossplot to be plug&play. But more extensible.

[stared]

@sebastienlange Do you want to contribute it to #60?

[sebastienlange]

@stared I would love to participate but I'm a bit overloaded right now. In which timeframe would you like to implement it? What kind of contribution do you expect? I'm particularly interested in visualizing intermediate activation in CNN!