Disable colored output on a dumb terminal (#3552)

Enable colored output by default only when the TERM environment variable
is set to a value other than dumb, as git and grep do, so that users no
longer need to pass -M or set NO_COLOR on terminals that cannot display
escape sequences. Colored output is now disabled not only for TERM=dumb
but also when TERM is unset or empty. The -C option still enables colors
in all these cases. On Windows, colored output is still decided by
ANSICON and the console mode, and TERM is not checked.

jq.1.prebuilt

@@ -115,7 +115,7 @@ Output the fields of each object with the keys in sorted order\.
 \fB\-\-color\-output\fR / \fB\-C\fR and \fB\-\-monochrome\-output\fR / \fB\-M\fR:
 .
 .IP
-By default, jq outputs colored JSON if writing to a terminal\. You can force it to produce color even if writing to a pipe or a file using \fB\-C\fR, and disable color with \fB\-M\fR\. When the \fBNO_COLOR\fR environment variable is not empty, jq disables colored output by default, but you can enable it by \fB\-C\fR\.
+By default, jq outputs colored JSON if writing to a terminal\. You can force it to produce color even if writing to a pipe or a file using \fB\-C\fR, and disable color with \fB\-M\fR\. When the \fBNO_COLOR\fR environment variable is not empty, or the \fBTERM\fR environment variable is unset, empty, or set to \fBdumb\fR, jq disables colored output by default, but you can enable it by \fB\-C\fR\.
 .
 .IP
 Colors can be configured with the \fBJQ_COLORS\fR environment variable (see below)\.