Added savefig for saving plots as files

[mickvanhulst]

In terms of documentation, I don't want to ruin the style you've created, so feel free to amend the following:

To save the plot, add save_img=True and add the file_name parameter with the directory, filename and file name extension (e.g. file_name='./folder_1/filename.png') as parameters when calling PlotLosses().

[stared]

Linking #14.
@mickvanhulst Ideas is nice, but code would need some refactoring.

1. There should be only one parameter (fig_path, if None, it does not produce a plot).
2. If you pass it, make sure to use only keyword arguments with default value None.

[mickvanhulst]

@stared Reformatted code, let me know if any other work is required.

[stared]

@mickvanhulst Finally, I was able to test it.

It works, thanks! I am merging that.