Fix output in corner cases

[nyancient]

This PR contains two separate bugfixes:

1. beep shouldn't be written to stdout when redirected, and
2. always erase the spinner on exit.

(1) is self-explanatory, but (2) might require some background. Currently, the spinner character is left on screen after the spinner exits if the next character written to stdout is a control character (newline, tab, etc.), but is erased if the next character written is printable. This inconsistent behavior looks like a bug to me.