Fix display on "dumb" terminals

Problem: brew does not respect "dumb" terminals.

When running brew from a dumb terminal, I have the following display:

% brew reinstall caffeine
==> Fetching downloads for: caffeine
⠋ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠋ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠙ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠙ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠚ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠚ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠞ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠞ Cask caffeine (1.1.4)                                                                                          Downloading 827.6KB/-------⠖ Cask caffeine (1.1.4)                                                                                          Downloading 827.6K
[...]

From man term(7):

   Older UNIX systems pre‐set a very dumb terminal type like ‘dumb’  or  ‘di‐
   alup’  on  dialup  lines.   Newer ones may pre‐set ‘vt100’, reflecting the
   prevalence of DEC VT100‐compatible terminals and personal‐computer  emula‐
   tors.

Those "dumb" terminals is are still useful nowadays. For instance,
they are the default value in Emacs shell or eshell. In those
terminals, we cannot use the ANSI escape sequences to move the cursor
around (but I guess, today all dumb terminals supports ANSI colors
escape sequences).

Solution: Detect whether brew is run from a dumb terminal and stop
displaying progress in that case.

The result is:

  % bin/brew reinstall caffeine
  ==> Fetching downloads for: caffeine
  ✔︎ Cask caffeine (1.1.4)
  ==> Uninstalling Cask caffeine

We only display the final status (green tick or red cross) since
nowadays dumb terminals seem to understand the color control sequences.