Feature: theme-related pages

[oott123]

Based on this idea.

Theme can define a list of pages it wants to render, and VuePress will render it during buliding.

I have wrote some docs along with the code in Chinese, please check it out in the code diff or here.

[meteorlxy]

I'm waiting for #277 to be merged to add this feature

[meteorlxy]

I don't think add the extra pages to options.siteData.pages is a good idea.

For example, if users want to v-for all the pages, they have to exclude the extra pages manually.

[ulivz]

Thanks!

I think we should avoid opening new file-level API for new features. all of them should be transferred to plugin so that we can only focus on designing plugin API.