Attempt to check whether output stream is a TTY before using color escapes.

This doesn't check full termcap attributes, but it checks with isatty(3).

Also, this follows the current #ifdef _WIN32 #else assume Unix #endif
pattern; this will need to be changed if more platforms are added.
Merge pull request #3 from silentbicycle/master

Attempt to check whether output stream is a TTY before using escapes