Implement CPI calculator

[mfirhas]

Command: /cpi

Calculate cpi to know how much you're getting poorer holding toilet papers.

Arguments:

• No args: show default picks of currencies for 5 years back.
• [currency] [from-date]: calculate how much cpi from from-date to now.
• [currency] [from-date] [to-date]: calculate how much cpi from from-date to to-date.