Detect whether or not the terminal supports color.

Detect whether or not the terminal supports color.

Before v0.4, term used to return `Ok(true)` if something succeeded,
`Ok(false)` if the operation was unsupported, and `Err(io::Error)` if
there was an IO error. Now, it returns `Ok(())` if the operation
succeeds and `Err(term::Error)` if the operation fails. If the operation
is unsupported, it returns `Err(term::Error::NotSupported)`. This means
that, if `op` is unsupported, `try!(term.op())` will now return an error
instead of silently failing (well, return false but that's effectively
silent).

Fixes #2338