Support specifying directories for template, secret template paths

[montmanu]

Is your feature request related to a problem?

1. Managing all k8s resources in a single file becomes difficult to maintain for larger projects
2. Executing drone-gke multiple times for each individual file seems a bit excessive

Describe the solution you'd like

An API to drone-gke that is compatible with kubectl apply's -f / --filename argument:

-f, --filename=[]: Filename, directory, or URL to files that contains the configuration to apply

Describe alternatives you've considered

1. Managing all resources in a single file
2. Executing drone-gke once per manifest file

Additional context

Might be worth considering in combination with #104

[benweint]

I've also wanted this recently.

Supporting kustomize / apply -k might be a nice way to achieve this, with the side benefit of allowing folks to avoid templating their K8s manifests and build said manifests locally more easily.