Export-PipeScript should allow conditional builds

[StartAutomating]

In a number of places throughout PipeScript, [ValidateScript] and [ValidatePattern] are used to determine validity of a thing, given an input.

Export-PipeScript should use this pattern to allow builds to be conditional.

• A *.build.ps1 should be considered valid if the commit matches [ValidatePattern]
• A *.build.ps1 should be considered valid if it has a [ValidateScript] that returns true
• A *.build.ps1 with no validation should always be considered valid

Handling these three variations on a theme allow for long running/slow changing builds to be run less frequently.

For example, a build could run only when "test" is present in the CI message.

Or, more interestingly, a portion of build could run only if the last checkin was older than a timeframe.