feat: Opt-out dep-builds and repo-updates (#463)

* Improve code organization

To make sure it is still readable after upcoming changes to helmfile

* feat: `helmfile deps` to update dependencies of all the local charts

Resolves #450

* feat: helmfile updates repos and build deps by default

But not update deps. Use `helmfile deps` to update deps, and provide `--skip-deps` to skip updating repos and builds deps in sync/diff/apply/template

Resolves #415

* Improve integration test coverage

pkg/app/context.go

@@ -0,0 +1,33 @@
+package app
+
+import "github.com/roboll/helmfile/state"
+
+type Context struct {
+	updatedRepos map[string]struct{}
+}
+
+func NewContext() Context {
+	return Context{
+		updatedRepos: map[string]struct{}{},
+	}
+}
+
+func (ctx Context) SyncReposOnce(st *state.HelmState, helm state.RepoUpdater) []error {
+	var errs []error
+
+	allUpdated := true
+	for _, r := range st.Repositories {
+		_, exists := ctx.updatedRepos[r.Name]
+		allUpdated = allUpdated && exists
+	}
+
+	if !allUpdated {
+		errs = st.SyncRepos(helm)
+
+		for _, r := range st.Repositories {
+			ctx.updatedRepos[r.Name] = struct{}{}
+		}
+	}
+
+	return errs
+}