tsm: support background color lightening via blink attribute

According to
http://misc.flogisoft.com/bash/tip_colors_and_formatting#terminals_compatibility
several terminal emulators (Linux TTY included) use blink to set
high-intensity background color. Rxvt doc states it as well:
https://github.com/exg/rxvt-unicode/blob/master/README.FAQ#L319-L324