Allow prerelease modules for configuration

[msftrubengu]

This PR allows a configuration file to have a boolean allowPrerelease directive to use prerelease DSC resources.

properties:
  configurationVersion: 0.2
  resources:
    - resource: FakeModule/FakeResource
      directives:
        version: '1.0.0-preview'
        allowPrerelease: true
      settings:
        Args: 'args'

Internally, allow prerelease is used only for a module that it needs to get install. When is enabled, the processor will use -AllowPrerelease to the Find-DscResource command.

It also fixes and infinite loop in SplitIntoLines. If the found offset is the one of "\r\n", it uses the same offset for pos in find_first_of, so it keeps returning the offset making nextOffset - currentOffset = 0

I don't know why VisualStudio decided to change a character...

Microsoft Reviewers: Open in CodeFlow

[denelon]

Do we need to modify the JSON schema for this?
Or create a 0.2 JSON schema?

[msftrubengu]

I can create the schema in another PR.