Fix #481 - Use $SHELL instead of sh in execute action

Note that $SHELL only points to the default shell instead of the current
shell. If you're on a non-default shell, you might want to override the
value like follows.

  SHELL=zsh fzf --bind 'enter:execute:echo $ZSH_VERSION; sleep 1'

src/terminal.go

@@ -720,7 +720,11 @@ func quoteEntry(entry string) string {
 
 func executeCommand(template string, replacement string) {
 	command := strings.Replace(template, "{}", replacement, -1)
-	cmd := exec.Command("sh", "-c", command)
+	shell := os.Getenv("SHELL")
+	if len(shell) == 0 {
+		shell = "sh"
+	}
+	cmd := exec.Command(shell, "-c", command)
 	cmd.Stdin = os.Stdin
 	cmd.Stdout = os.Stdout
 	cmd.Stderr = os.Stderr