text: Title: add `nrows`
  Body: - expose a new argument `nrows=None`
      + default: auto-detect terminal height with some sensible default such as `20` (similar to `ncols`)
  - hide bars `if n < total and pos >= nrows`
      + completed/`close()`d bars will ignore `nrows` and just print according to `leave`
  - [ ] TODO: link related issues
  
  This fixes issues where there are more nested bars than room for on a screen.
  
  Cross-platform auto-detection compatibility will be at least as annoying as is for `ncols`.