Allow fallback to be true

[colinking]

This is a follow-up PR to: #13

In that PR, fallback could only be a function or false. If it was set to true, it would produce an error because it would attempt to execute it as if it was a function:

  Error thrown in test:

  TypeError {
    message: 'options.fallback is not a function',
  }

This means that upstream consumers would need to replace fallback=true with fallback=undefined, which isn't a great experience.

This PR just updates terminal-link to produce the default fallback if fallback=true, and updates the associated documentation.