Debugger: HTML mode is off with CLI SAPI

headers_list() is always empty with PHP_SAPI === 'cli'. PHP manual says: "Headers will only be accessible and output when a SAPI that supports them is in use".
Dumper: html mode detection is the same as Debugger's one
Dumper: terminal 'screen' is color-capable too
Dumper: terminal color dump only if STDOUT is a terminal

This avoid printing escaping sequences in cases of redirection like:
  php dump.php > output.log
  php dump.php | less