cargo web uses color even when not supported

[jgarvin]

Color should only be used when the output is a tty and when the terminal supports color. rustc/cargo normally understand this, but when invoked through cargo web build I get a lot of this:

�[0m�[0m�[1m�[32m Compiling�[0m webidl v0.8.0

This is annoying when you are redirecting stdout/stderr to a log file for example.