add ttyless support

[Elan Ruusamäe]

i would like to parse output of poldek errors, but it prints ansi escapes when run from terminal
and if not ran from terminal it says "not a tty"

problem 1:
echo "install blabla" | script test.log -c "sudo poldek"
and inspect test.log

for that --color=no option would be useful

problem 2:
similarily can't use it this way:
echo "install blabla" | sudo poldek | tee test.log

test.log would contain "not a tty" error

so for problem 1, add --color (or something similar with yes,no,auto values)
and for problem 2, make it work instead of giving "no a tty" error