Fix legend overlapping with lines on causal impact plots

[drbenvincent]

In a number of situations, the legend is overlapping with data in the causal impact plots. Here's an example:

• Create a kwarg in the plot method which allows users to specify that they want the legend outside the plot. It might take some experimentation to get this right. For example, does it look best to put the legend on the side of the plots (but this may look strange) or perhaps under the bottom?

[anevolbap]

Adding something like this should work:
sns.move_legend(ax[0], "upper left", bbox_to_anchor=(1, 1))

Maybe figsize can be adjusted to make wider as before 🤔