feat($plugin-blog): Allow the permalink for blog posts to be configured.

[seifertm]

Summary

The VuePress blog plugin provides an opinionated default for the URL of blog posts. As of v1.0.0-alpha.20 changing the permalink URL of blog posts is only possible via passing a custom "pageEnhancer" to plugin-blog. That pageEnhancer basically overwrites page.frontmatter.permalink. This workaround config.js looks similar to:

module.exports = {
  plugins: {
    '@vuepress/blog': {
      pageEnhancers: [{
        when: page => page.type === 'post',
        frontmatter: {permalink: '/custom/permalink/:slug'}
      }]
    }
}

To simplify the solution, this PR adds a new configuration option permalink to the blog plugin that allows to configure the permalink URL for blog posts. The new configuration option uses the same default as previously:

module.exports = {
  plugins: {
    '@vuepress/blog': {
      permalink: '/custom/permalink/:slug'
    }
}

The corresponding plugin documentation in English language was updated.

plugin-blog does not seem to have any tests that can be updated (please correct me, if I am wrong).

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:

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

• Yes
• No

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:

[ulivz]

LGTM