Check the COLORTERM variable for 16M color support (#75)

According to https://gist.github.com/XVilka/8346728#detection and empirical tests on Konsole,
when `COLORTERM=truecolor`, 24-bit color support is guaranteed to work.

index.js

@@ -88,6 +88,10 @@ function supportsColor(stream) {
 		return /^(9\.(0*[1-9]\d*)\.|\d{2,}\.)/.test(env.TEAMCITY_VERSION) ? 1 : 0;
 	}
 
+	if (env.COLORTERM === 'truecolor') {
+		return 3;
+	}
+
 	if ('TERM_PROGRAM' in env) {
 		const version = parseInt((env.TERM_PROGRAM_VERSION || '').split('.')[0], 10);