Only emit terminal codes to terminal (and don't require say)

[dannysauer]

Issue #, if available:
Fixes #220
Fixes #239

Description of changes:

• Check for presence of tput and for stdout being a terminal before emitting terminal control codes.
• Remove unnecessary dependence on say (which was removed: git/git@5b893f7)

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[cahrens]

It would be great to get some eyes on this PR, as we are sporadically hitting the problem of say not being present (we also see the terminal warning, but that doesn't cause a failure).

[dannysauer]

Is @mtdowling still affiliated with this project? Your name is all over the blame from seven years ago. :)

Trying to find some reviewers.

[sparr]

I pushed a commit to use command instead of which, but otherwise this is a great change that resolves outstanding test failures.

[kdenhartog]

for those who are on MacOS and still encountering this problem, you'll need to use the make install method of installation. The brew cask has not been updated yet.

[irgaly]

Homebrew's git-secrets v1.3.0 still uses say command, so macOS users encounters VoiceOver's voice from your speaker when you run git secrets --install command.

brew link git-secrets --HEAD will run make on master branch and install git-secrets that does not contains say command problem.