Display winning portfolios from given time period

[gayatribir]

As a stock investor, I want to see the list of winning portfolios from a given period so that I can decide and invest more wisely.

Acceptance Criteria:

• [ ] Add a /portfolio command which takes one parameter.
• [ ] When the command is entered, the user should see the list of portfolios that have performed best.
• [ ] The command should take a parameter only in the format of yyyy-mm (all numbers).
• [ ] When the parameter is not passed, the command should not be entered.
• [ ] The command should show the ticker symbol, number of shares, start value, end value, and percentage gain.
• [ ] If there is no result for this command, display appropriate message on the screen.