* Add "--namespace" to helm lint

Keep lint syntax as close as possible to "helm install" resp. "helm
upgrade", so that one only needs to change the command.

See #2036

* Align lintCmd struct

* Add "--set" and "--values" to helm lint

Keep lint syntax as close as possible to "helm install" resp. "helm
upgrade", so that one only needs to change the command.

Closes #2495,#2036

* Reuse strict parameter, when rendering during lint

We want to see the rendering fail, if we missed a value, so we reuse
"--strict".

See #2495,#2036

* Fix lint unit test

See #2495,#2036

* Update docs