Support configuration file

[yhatt]

This PR will support configuration file like marp.config.js or .marprc (+ with extensions .json, .yml, .yaml, and .js), powered by cosmiconfig.

You can specify CLI options through config file declaratively. For example, a below .marprc file will instruct to output pdf with using uncover theme.

{
  "theme": "uncover",
  "pdf": true
}

The biggest benefit is a customization of Marp Core's constructor option by options. These are too complex to be config through CLI, so you must use configuration file.

# .marprc.yml

options:
  markdown:
    breaks: false
  emoji:
    shortcode: false
    unicode: false

html: # Setting HTML whitelist (Marp core's feature)
  a:
    - href
    - target
    - title
  br: []

Some flags may not be able to use because they will override by marp-cli.

If you are using configuration file with .js extension, you can swap the conversion engine to Marpit based engines. The below example is using pure Marpit framework.

// marp.config.js
const { Marpit } = require('@marp-team/marpit')

module.exports = {
  engine: Marpit,
  options: {
    lazyYAML: true,
  },
}

Of course you can specify the path to configuration file by CLI --config (-c) option.

# Convert markdown in "~/slideshow" with specifed configuration
marp ~/slideshow -c ~/slideshow/config.json

ToDo

• Add tests

[yhatt]

Update engine support to be able to specify module name by string. In addition, it becomes to be able to parse marpBrowser section of package.json as browser context JS. (= Marp.ready(), that has already defined in @marp-team/marp-core)

It means that you can use CLI directly while developing Marpit based engine. cosmiconfig will read marp section on package.json, so you can develop an original engine with this config:

{
  "private": true,
  "scripts": {
    "build": "marp"
  },
  "devDependencies": {
    "@marp-team/marpit": "^0.0.14",
    "@marp-team/marp-cli": "^0.0.5"
  },
  "marpBrowser": "lib/browser-context.js",
  "marp": {
    "engine": "./lib/marpit-based-engine.js"
  }
}

export default class MarpitBasedEngine extends Marpit {
  // ...
}