Markdown control and layout fixes

• New: optional markdownFunctions.block & markdownFunctions.inline
  configuration options. This allows you to use a custom markdown parser
  for rendering slides (beyond setting basic markdownIt options).
• Breaking: rename slide markdown filters
  slideMDownBlock() to slideMarkdownBlock() &
  slideMDownInline() to slideMarkdownInline() for clarity.
• Breaking: Update dependencies
• New: Show progress bar in speaker view
• Fix: Maintain slide aspect ratios in portrait mode slideshow view
• Fix: Remove extra wrapper elements on some slide types

0.3.0

• Breaking: Image slides use slide.credit rather than slide.cite
  for displaying photo credit.
• Breaking: Remove new pen templates and pen: new behavior,
  since the use-case is simple to handle with known pens.
• Breaking: getKnown() global function is removed in favor of buildSlides()
• Fix bug where <pen-slide> and <demo-slide>
  now extend <embed-slide> directly,
  including necessary styles.
• Add slideDeck collection (name can be configured).
• Pre-build slide data into $data.slideDeck
  with known slides/series references resolved,
  and slide layout determined (when not provided).
  This can be configured with an additional build function.
• Pre-build cited resources and CodePen demos into
  $data.slideResources.cite and $data.slideResources.pens.
  Both are filtered to include unique links,
  and return an array of inline markdown strings.
• Support explicit slide layouts with slide.layout property.
• Support series references with slide.series property
  (similar to slide.known but for referencing multiple slides).
• Add <render-slide> component for rendering slide data
  with a built-in layout.
• Simplify <pen-slide> to remove extra (never-documented) features,
  and add support for CodePen 2.0 pens.
• All slide types accept a markdown slide.cite property.
• Slides that have slide.source, slide.name, or slide.url
  will generate default slide.cite data
  if not provided explicitly.
• Quote slides will use either slide.img or slide.avatar data
  for rendering an avatar next to the quote.
• Add <source-slide> to display a resource with optional avatar/image.
  Since source information is allowed on all slide layouts,
  this is only the default when no other data is provided.
• Add <error-slide> to stop build failures
  when a slide component is rendered without slide data,
  and to render a json string when the data isn't recognized.
• Default slide deck theme supports both light and dark modes out of the box,
  using light-dark() so that control is left to the page.

Configuration Options…

• collectionName changes the name of the slideDeck collection,
  including every page with slides data.
• buildFunction (when provided)
  will be called on each slide during the build process.
  Use this to set additional layout types
  or make other slide-data alterations.
• known.slides (default knownSlides)
  and known.series (default knownSeries)
  establish where to look for slide and series source data.

Provided filters…

• Remove placeHolder() (use slideImg() without a path).
• Add buildSlides() for building out slide decks from any slide data.
• Change slideStyles() so that color, background, and mode
  are always allowed.

v0.2.0

What's Changed

• Add default theme by @dvdherron in #2
• Update docs by @jamesnw in #1

New Contributors

• @dvdherron made their first contribution in #2
• @jamesnw made their first contribution in #1

Full Changelog: v0.1.0...v0.2.0

Initial draft

Initial features for testing