Support dark color-scheme

[viztor]

currently, the taskbook doesn't look as intended in dark-themed terminal interfaces.

the task list isn't legible, and many other colors ain't adaptive to the change of the terminal.
maybe should use standard terminal color variables, therefore it would adapt itself.
a more complicated solution would be offering color-scheme option to the user.

[rjoydip-zz]

I think PR #34 is appropriate for your case.