GBTC premium as a percentage of the bitcoin price

Plots the GBTC premium as a percentage of the bitcoin price at the close. It also includes the annual management fee (2% at the moment). It accrues on a daily, weekly or monthly basis depending on the chart resolution. (intraday not supported) The amount of bitcoin per gbtc share and the annual fee may be changed by Grayscale in the future. Those values can be easily update in the script settings.