Fix `tmux` text being always red

[rico-chet]

tmux (https://github.com/tmux/tmux/wiki) is a terminal multiplexer, like screen. It spawns its terminals with argv[0] prefixed with a '-', thus breaking stderred's early return on bash encounter. This leads to normal text being always red.

When PROGRAM_NAME matches the '-%s' format pattern, treat it as a shell and return early.

Refer to https://github.com/tmux/tmux/blob/c9d482ab489d5d57d481858091608ee1b32e46ab/window.c#L993 and to https://github.com/tmux/tmux/commit/22d1b9412e52c98cbdf52a3a5185a416c6d26c64.

Fixes https://github.com/sickill/stderred/issues/52