Dot output for labeled list of value + expose dot context

In dot output, I added dump_list_XXX functions to allow the dump of several values in one graph, with the ability to add labels to identify them on the graph. The dump_XXX functions calls these new dump_list_XXX functions with a list of one element, with no label.

I also exposed the context and created a context class, of type context_t, so that the user of the library can inherit from the default class to make some changes in the output (colors, styles, ...).