Switch from 'term' to 'termcolor' crate

The 'term' crate is unmaintained:

Stebalien/term#93

The 'termcolor' crate (by @BurntSushi) is maintained, provides a higher
level abstraction which is a better fit for what we're doing, and has
fewer (transitive) dependencies:

https://github.com/BurntSushi/termcolor