Add ability to set context for the diff

Previously, context flag was set to something sane via: https://github.com/mumoshu/terraform-provider-helmfile/commit/1a477d9d45c5a82847128c6b1c1614f36bc6ddad.  Somewhere this commit was dropped.  It'd be nice to add this functionality as the current diff view is very noisy.