minor tweaks for API consistency in 1.x

[jenshalm]

This is the final planned coding PR for 1.0. It's simply a final batch of changes for names of types and methods for consistency in naming conventions or for better API docs (e.g. moving some types into companions).

Renaming

• RenderContext -> Formatter.Context
• BinaryPostProcessorBuilder -> BinaryPostProcessor.Builder
• Font.webCSS -> .withWebCSS
• Font.embedResource -> .withEmbeddedResource
• renderMessages -> withMessageFilter (in several config types)
• failOnMessages -> withMessageFilter (in several config types)
• move MessageFilter from laika.ast to laika.config
• rename ConfigError types to avoid double Error postfix (e.g. ConfigError.DecodingError)
• turn ParserError and RendererError into traits and use more detailed errors as concrete types:
  • InvalidInput
  • InvalidConfig
  • InvalidElements

New API

• Add Helium.extendWith(Helium => Helium)