1.0.0-beta.20

• Breaking change: Removed the IDocument.GetParent(), IDocument.HasChildren(), IDocument.GetDescendants(), and IDocument.GetDescendantsAndSelf() extension methods
  (IDocument.GetChildren() still remains since it's fetching actual metadata values). Instead, the appropriate tree concept can now be accessed via an implementation
  of the IDocumentTree<TDocument> interface. This change may break navigation, so Outputs or OutputPages should be considered for generating navigation instead.
• Added an OutputPages property to the engine and execution context to make it easier to filter outputs by "pages" (by default, documents with a destination path ending in ".htm" or ".html").
  This is what you should use to generate navigation going forward.
• Added Outputs.Get() and OutputPages.Get() to support getting a single document from those collections by destination path, which is faster than globbing.
• Added a IDocumentTree<TDocument> interface to encapsulate different kinds of document tree traversal logic.
• Added a DocumentMetadataTree<TDocument> implementation to represent document trees as the result of metadata containing child documents.
• Added a DocumentPathTree<TDocument> implementation to represent document trees as the result of file paths.
• Added a IEnumerable<TDocument>.AsMetadataTree() extension method to get a DocumentMetadataTree<TDocument> instance that creates a tree from document metadata containing child documents.
• Added a IEnumerable<TDocument>.AsDestinationTree() extension method to get a DocumentPathTree<TDocument> instance that creates a tree from document destination paths.
• Added a IEnumerable<TDocument>.AsSourceTree() extension method to get a DocumentPathTree<TDocument> instance that creates a tree from document source paths.
• Added a new FilteredDocumentList<TDocument> return type for IEnumerable<TDocument>.FilterDestinations() and IEnumerable<TDocument>.FilterSources() calls
  (including Outputs[string[] patterns]) which implements the new IDocumentTree<TDocument> and lets you treat the resulting filtered documents as a tree from the filter return.
• Added the IDocumentTree<TDocument> interface to IPipelineOutputs with default implementations that operate on document destination paths. This means you can call
  methods like Outputs.GetChildren(doc) to get all the children across all pipelines of the given document, etc.
• Added a new IndexFileName setting to control the default file name of index files (defaults to index.html).
• Added a new PageFileExtensions setting to control the default file extensions of "pages" for things like OutputPages filtering and link generation (defaults to ".html" and ".htm").
• Added a new constructor to the SetDestination module that will change the destination of documents to the first value of the PageFileExtensions setting (default of ".html").
• Added a new MinimumStatiqFrameworkVersion key to perform a check for the minimum allowed version of Statiq Framework. If this is set to something higher than the current version
  of Statiq Framework, an error will be logged and execution will stop. Any setting that starts will this key will be considered, so it's recommended the use of this key be
  suffixed with a unique identifier to avoid conflicts between components (for example MinimumStatiqFrameworkVersion-MySite).
• Refactored settings and configuration implementations. You shouldn't notice anything usage-wise, but keep an eye out for anything that doesn't work as expected