First class .Build.Date

[ehotinger]

Purpose of the PR:

First classes {{.Build.Date}}, which defaults to time.Now().UTC() with yyyyMMdd-HHmmssz format.

Fixes #161

/cc @SteveLasker

[sajayantony]

Once this is rolled out, I think we need to enable this format to be specified.

[ehotinger]

@sajayantony At that point you should just do {{ now | date "layout" }} which is already supported. I think it makes sense to first class a pre-defined date string so that people don't have to do that work, but once you start trimming/formatting/etc. you should be diving a bit deeper.

[northtyphoon]