feat($markdown): refine markdown api

[shigma]

Summary

What kind of change does this PR introduce? (check at least one)

• Bugfix
• Feature
• Code style update
• Refactor
• Docs
• Build-related changes
• Other, please describe:

If changing the UI of default theme, please provide the before/after screenshot:

Does this PR introduce a breaking change? (check one)

• Yes
• No

If yes, please describe the impact and migration path for existing applications:

The PR fulfills these requirements:

• When resolving a specific issue, it's referenced in the PR's title (e.g. fix #xxx[,#xxx], where "xxx" is the issue number)

You have tested in the following browsers: (Providing a detailed version will be better.)

• Chrome
• Firefox
• Safari
• Edge
• IE

If adding a new feature, the PR's description includes:

• A convincing reason for adding this feature
• Related documents have been updated
• Related tests have been updated

To avoid wasting your time, it's best to open a feature request issue first and wait for approval before working on it.

Other information:

feat: support env.loader

User can get access to markdown-loader using env.loader. Example:

module.exports = (md, options) => {
  md.renderer.rules.my_rule = (tokens, index, options, env) => {
    const { src } = tokens[index]
    env.loader.addDependency(src)
    if (!fs.existSync(src)) {
      env.loader.emitWarning(`File Not Found: ${src}.`)
    }
  }
}

refactor: refine link bahavior

Every token is an instance of Token. Its prototype should not be removed, otherwise other plugins will run into error if any instance method is called (such as token.attrIndex('href')).

[ulivz]

Did the example have a typo?

env.emitWarning(`File Not Found: ${src}.`) ---> env.loader.emitWarning(`File Not Found: ${src}.`)