Plugin to Generate Feature Log Ratios By Textual Searching à la Qurro

For reference: we're planning on rolling this into Qurro as a separate method (distinct from the visualizers). Along with allowing the computation of log-ratios without making the user use the interactive visualization, this would have the advantage of facilitating these computations in a way that gets around max number sizes in JavaScript, as discussed in this issue and in this forum post.