Variable names in element labels for `plot_forest` should be optional

Currently, creating a forest plot with plot_forest uses <var name> <coordinate name> as the scheme for labels on the y-axis. This creates a slew of repeated variable names in the labels, which I would guess is almost always undesirable (particularly when you have nice element labels as coordinates in the dataset):

There should be excluded by default, and turned on with a include_var_names=True (or similar) argument.

Alternatively, the first coordinate could include the variable name, and then exclude it for subsequent ones.

[fonnesbeck]

… and in Bokeh

[fonnesbeck]

How about something like this (using the mpl_plot_forest_ridge example):

here is the current behavior:

Got a PR ready to go if this works for everyone.