Support for --secret flag

[bradrydzewski]

The --secret flag should land soon and could be very useful for teams that are currently using --build-args and would like something a bit more secure (that doesn't print to the console). docker/cli#1288

[tboerger]

This can be added after the upgrade to 18.09 ;)

[wszychta]

@bradrydzewski @tboerger
I have created PR with support for secrets - #332
It is working fine with current master branch and in my opinion it can be merged.
If you have any suggestions please leave your suggestions in PRs comments.