Adds flags for installing knative a la carte

[psschwei]

Signed-off-by: Paul S. Schweigert [email protected]

Changes

Adds command line flags --install-serving and install-eventing to
enable users to install only one Knative component (i.e. only serving).
Using both flags will install both components (which is also the default
behavior when no flag is passed).

Fixes #224
Release Note

Adds command line flags to let users specify which Knative component(s) they want to install. For example, `kn quickstart kind --install-serving` would create a Kind quickstart cluster with only Knative Serving installed. No flag is required, and `kn quickstart minikube` would install both Serving and Eventing, as is currently done. 

[knative-prow]

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[csantanapr]

this looks good
/lgtm