Adding logarithm of eqity curve option

[FilJed]

Found no disscussion about opportunity of displaying logarithm of equity curve.

Given that it's possible to retrieve equity data from stats['_equity_curve'] I can plot it myself. Or even change code to plot through bt.plot(). But it would be nice have it within backtest.py framework.

Reason: When testing long time distances, it's hard to see how strategy performmce at early stages (see picture attached). Zooming has no autoscaling as well. By plotting logarithm of eqity curve it's more obvious to get this insights and it is also connected to utility function.

[FilJed]

Upd: there was unanswered question here:
#404