Add suggestions for misspelled commands

[ffaf1]

To reproduce:

• type toot notfications (notice misspelling)

What happens:

• Unknown command 'notfications'
  Run toot --help to show a list of available commands.
  

What I would like to see:

• Unknown command 'notfications'
  Maybe you meant `notifications`?
  Run toot --help to show a list of available commands.
  

It would also be useful for #401.