Adding list of indices and kwargs for ratio_plot

[aretaon]

Is your feature request related to a problem? Please describe.
With the current version, multiple highlights are supported in the volcano plot functions but not in the ratio-ratio plot.

Describe the solution you'd like
An identical approach to using multiple highlights should be taken for the ratio plots (i.e. it should be possible to supply a list of highlight-indices and kwargs). This should be unified with the existing functions for the volcano plot