LogFormatter: allow color support when sys.stderr.isatty() == False

LogFormatter:
https://github.com/tornadoweb/tornado/blob/c447875a1058f4768c2995ddd9ebb0eaddd3f32e/tornado/log.py#L81
... has very nice support for color logging via curses (or colorama).

My issue is with the test sys.stderr.isatty() in:

https://github.com/tornadoweb/tornado/blob/c447875a1058f4768c2995ddd9ebb0eaddd3f32e/tornado/log.py#L55-L71

The LogFormatter might be able to be attached to a handler which is not writing to stderr type stream, but to some other TTY-capable device, which I have in my case. The syserr TTY test prevents attaching the LogFormatter to such logger handler and having color support.

If I monkey-patch _stderr_supports_color() to lambda: True, and make sure I make a call to curses.setupterm(fd=...), passing the fd of my TTY-capable stream, then I can log with the color support from LogFormatter, which is nice.

Maybe the best fix is to change color: True to None, but default based on the detection above, or if set to True explicitly by the caller, honour that.

Start in tornado/log.py at LogFormatter and _stderr_supports_color(), then review the existing stderr TTY check and the caller-provided color option. Compare how curses.setupterm(fd=...) is used for a TTY-capable stream. Done means an explicitly enabled color formatter can work with a non-stderr TTY without regressing default detection.