Style: Better formatting of usage summary

[jschuur]

• [ ] add commas to token numbers
• [ ] always show at least 2 digits after the decimal points
• [ ] align the currency periods even when it's more than 1 digit before the dollar amount
• [ ] show session usage too

Turn this:

Session cost: $0.0008 (377 tokens)
Total cost:   $10.1 (57619 tokens)

Into:

Session cost: $ 0.0008 (377 tokens)
Total cost:   $10.10   (57,619 tokens)