Make plot_histogram & iplot_histogram similar

[ajavadia]

What is the expected enhancement?

1- Make the function signature the same for both functions
2- Make the iplot_histogram default when using jupyter and connected to internet.

How to do this?

In order to easily switch between the plot_histogram and iplot_histogram functions, both need some changes.
1- The plot_histogram function must be able to plot multiple histograms in one figure, just like iplot_histogram does.

2- We have to stay backward-compatible. Therefore, the iplot_histogram(counts) should work.

So I'm thinking we use this function signature?

[i]plot_histogram(counts = {'000': 4, '010': 10}) # plot a simple counts histogram
[i]plot_histogram(counts = [{'000': 4, '010': 10}, 
                            {'000':8, '001': 4, '010': 9}]) # plot 2 histograms in one fig
[i]plot_histogram(counts = [{'000': 4, '010': 10}, {'000':8, '001': 4, '010': 9}],
                             legend = ['simulator', 'device']) # plot 2 histograms w legend
iplot_histogram(counts, legend, options) # the fancy options of the interactive plotter

More details

iplot_histogram
In the above, I'm proposing splitting the executions_results arg into a counts arg and a legend arg. I don't think we need the device thing at all.

plot_histogram
plot_histogram has a number_to_keep arg which i think should be absorbed in options. It will be a breaking change, but it does not affect as many users as the regular counts arg.