When to support Import Code Snippets

[John60676]

this will help manage common code snippets

[meteorlxy]

The code snippets support of v1 is not so perfect. And it's still marked as beta in v1 docs:

An enhanced version comes from this PR: vuejs/vuepress#1336 , and this one may be implemented in v2.

[meteorlxy]

Finally we decided to support this proposal:

vuejs/vuepress#538 (comment)