feat: helmfile as a go library (#639)

* feat: helmfile as a go library

This removes almost all the dependencies from the helmfile core logic to urfave/cli. `main.go` is now a thin wrapper around the core logic implemented in `pkg/app`.

pkg/app/ask.go

@@ -0,0 +1,33 @@
+package app
+
+import (
+	"bufio"
+	"fmt"
+	"log"
+	"os"
+	"strings"
+)
+
+// Copyright (c) 2017 Roland Singer [roland.singer@desertbit.com]
+//
+// Shamelessly borrowed from @r0l1's awesome work that is available at https://gist.github.com/r0l1/3dcbb0c8f6cfe9c66ab8008f55f8f28b
+func AskForConfirmation(s string) bool {
+	reader := bufio.NewReader(os.Stdin)
+
+	for {
+		fmt.Printf("%s [y/n]: ", s)
+
+		response, err := reader.ReadString('\n')
+		if err != nil {
+			log.Fatal(err)
+		}
+
+		response = strings.ToLower(strings.TrimSpace(response))
+
+		if response == "y" || response == "yes" {
+			return true
+		} else if response == "n" || response == "no" {
+			return false
+		}
+	}
+}