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`.

cmd/deps.go

@@ -1,37 +0,0 @@
-package cmd
-
-import (
-	"github.com/roboll/helmfile/pkg/app"
-	"github.com/roboll/helmfile/pkg/argparser"
-	"github.com/roboll/helmfile/pkg/helmexec"
-	"github.com/roboll/helmfile/pkg/state"
-	"github.com/urfave/cli"
-)
-
-func Deps() cli.Command {
-	return cli.Command{
-		Name:  "deps",
-		Usage: "update charts based on the contents of requirements.yaml",
-		Flags: []cli.Flag{
-			cli.StringFlag{
-				Name:  "args",
-				Value: "",
-				Usage: "pass args to helm exec",
-			},
-		},
-		Action: func(c *cli.Context) error {
-			return VisitAllDesiredStates(c, func(state *state.HelmState, helm helmexec.Interface, ctx app.Context) (bool, []error) {
-				args := argparser.GetArgs(c.String("args"), state)
-				if len(args) > 0 {
-					helm.SetExtraArgs(args...)
-				}
-
-				errs := state.UpdateDeps(helm)
-
-				ok := len(errs) == 0
-
-				return ok, errs
-			})
-		},
-	}
-}