GitHub action for converting markdown files to a combined ePub file.

Sample of the action with the required inputs:

- name: Create ePub
  uses: harrymaynard/markdown-to-epub-action@v1
  with:
    markdownFiles: |-
      test-data/publication.md
      test-data/chapter-*.md
    title: My Book
    author: John Doe

• markdownFiles: (required) Multi-line string containing markdown files to include. These can be indivitual files or globs.
• title: (required) Title of the book.
• author: (required) Author name.
• publisher: (optional) Publisher name. If not specified, none is included in the ePub file.
• cover: (optional) The book's cover image. This can be a URL (https://example.com/cover.jpg) or a path to the file located inside the repository (images/cover.jpg). If not specified, none is included in the ePub file.
• version: (optional) Version of the ePub template to use. This can be either 3 or 2. Default is 3 if not specified.
• lang: (optional) Language code for the book. Default is en if not specified.
• tocTitle: (optional) Override text for the table of contents. Default is Table of Contents if not specified.
• hideToC: (optional) Choose weather to include the table of contents in the ePub file. Default is false if not specified.
• output: (optional) File name of the output ePub file. Default is book.epub if not specified.