Support conditions in tags

[ArtAndreev]

I wanted to add envconfig tag as explicit tag required for function Process:

- name: github.com/kelseyhightower/envconfig.Process
  tag: envconfig
  arg-pos: 1

As it stated in README.md, tag ignored is used, when field must not be processed.

So there is no way to define that either envconfig or ignored must be added to struct.

[tmzane]

I understood your use case, will try to implement the feature.