Optional theme support for ignteiui for angular projects.

[bazal4o]

Currently the Ignite UI for Angular project generates source with modified palette applied to the Ignite UI for Angular theme (see styles.scss). That's more like the custom theme option (like you'd see in other projects).

There should be an option available to use the default theme unmodified (provided as an option in step by step mode and command). The custom theme can remain the default selection as it is.