feat: plugin container

[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:

• Introduced a brand new multiple plugin: vuepress-plugin-container. Since custom containers are quite common in websites, it will help a lot if users have easy access to a container plugin.
• improved code reusability in VuePress.
• Updated related docs.

[ulivz]

Good work, but maybe we need to move "markdown-it-container" to container plugin.

[ulivz]

LGTM

[tedmiston]

@shigma @ulivz Thank you for creating this plugin. It seems there is a bug in vuepress v1.0.0-alpha.41 breaking on the markdown-it-container dependency not being present. This is resolved by installing the missing dependency. I have created a bug report issue #1387 but wanted to cross-link here for reference.