Feature Request: Plot percent price change on the y-axis

**Is your feature request related to a problem? Please describe.**

When I want to compare two stocks, I'm interested in seeing the % changes in those two stocks over a period of time. Currently when I plot two stocks in mplfinance (using a secondary y-axis), I can't see the day-to-day percent changes, I only see the day-to-day price changes, which can be on different axes and thus inaccurate to compare directly.

For example, if I want to compare SPY and GOOG over a time period, I want to see the percent changes between the two (for easy comparison) rather than their stock prices which are almost an order of magnitude apart.

**Describe the solution you'd like**

A plot option that lets you plot the % change and lets you specify the start value that the percent changes should be compared against.

No files or tests are identified in the issue. Start by reviewing mplfinance's plot-option API and current secondary-y-axis behavior, then determine how percent-change comparison and a configurable starting value should be specified. Done means the option produces comparable percent-change plots for symbols such as SPY and GOOG, with coverage for the new behavior.

Written by the indexing model from the issue text.