Add Sensitive Flag for conf tags.

[abraithwaite]

At Golab, I was watching the keynote by @ardan-bkennedy where he shared their Service template. He demonstrated their conf package which works almost exactly like ours:

https://github.com/ardanlabs/conf

One killer feature in that package which we're missing is a flag to disable logging of sensitive parameters. Let's add something to that regard!

[Pryz]

Could be really nice to have redacted flag which fills the data with some * :)