Add user-facing commands for quantitative analysis

[jaARke]

As we build out functionality for conducting quantitative analysis, we'll want to add commands to make that functionality available for bot users. This should be done iteratively, to reflect each of our steps toward a fully functional analysis engine. The steps, as I see them, are as follows:

• Add command for retrieving (and downloading) historic stock data over a given timeframe (this issue).
• Add command for prediction via LSTM (this issue).
• Add commands for prediction via other techniques (this issue).
• Add command for conducting all supported analyses and displaying all results.

These commands should each provide a well-formatted embedding to the user displaying data entries and/or prediction results, as well as any relevant performance metrics.