Only output color if $TERM is not dumb (#264)

* Only output color if $TERM is not dumb
Probably should use github.com/fatih/color
Broken, sorry, b/c I don't know how to integrate "go get" in Makefile
I'm not a go programmer. :(
See fatih/color#50
Visit respectmyterm.com to learn why
Thanks!

* remove dependency on isatty
isatty isn't really relevant here. All we really want
is to be able to control the color by setting TERM=dumb

log.go

@@ -3,15 +3,18 @@ package main
 import (
 	"fmt"
 	"log"
+	"os"
 )
 
 const (
-	debugLogFormat   = "DBG-direnv: %s"
-	defaultLogFormat = "direnv: %s"
-	errorLogFormat   = "\033[31mdirenv: %s\033[0m"
+	debugLogFormat          = "DBG-direnv: %s"
+	defaultLogFormat        = "direnv: %s"
+	errorLogFormat          = defaultLogFormat
+	errorLogFormatWithColor = "\033[31mdirenv: %s\033[0m"
 )
 
 var debugging bool
+var noColor = os.Getenv("TERM") == "dumb"
 
 func setupLogging(env Env) {
 	log.SetFlags(0)
@@ -24,7 +27,11 @@ func setupLogging(env Env) {
 }
 
 func log_error(msg string, a ...interface{}) {
-	logMsg(errorLogFormat, msg, a...)
+	if noColor {
+		logMsg(errorLogFormat, msg, a...)
+	} else {
+		logMsg(errorLogFormatWithColor, msg, a...)
+	}
 }
 
 func log_status(env Env, msg string, a ...interface{}) {

stdlib.go

@@ -43,7 +43,7 @@ const STDLIB = "#!bash\n" +
 	"  local color_normal\n" +
 	"  local color_error\n" +
 	"  color_normal=$(tput sgr0)\n" +
-	"  color_error=\"\\e[0;31m\"\n" +
+	"  color_error=$(tput setaf 1)\n" +
 	"  if [[ -n $DIRENV_LOG_FORMAT ]]; then\n" +
 	"    local msg=$*\n" +
 	"    # shellcheck disable=SC2059\n" +

stdlib.sh

@@ -41,7 +41,7 @@ log_error() {
   local color_normal
   local color_error
   color_normal=$(tput sgr0)
-  color_error="\e[0;31m"
+  color_error=$(tput setaf 1)
   if [[ -n $DIRENV_LOG_FORMAT ]]; then
     local msg=$*
     # shellcheck disable=SC2059