Chalk output previously contained color when the `TERM` environment
variable had the value `dumb`. Disable color for that situation.

Chalk already correctly disabled color when:

- The user passes `--no-color` [1], For example:

     `chalk version --no-color`

- The `NO_COLOR` environment variable is set. For example:

     `NO_COLOR=1 chalk version`

- stdout is not a tty. For example:

     `chalk version > /tmp/version.txt`

Closes: #162

[1] See also https://no-color.org/