Consider renaming `themedir/index.js`

[mdaffin]

#154 was recently merged but with the discussions about introducing a server-side plugin framework (being discussed in #198), we may want to consider renaming the themeDir/index.js in prepare.js L162 to something else if we decide we want to use that for the server side plugins instead. If we do this then we can treat the themeDir as just another plugin rather then having to special case it.

If we release the next version as is then this will be a lot harder to change in the future.

Proposals:

• enhanceApp.js - consistent with the .vuepress/enahnceApp.js.
• themeApp.js
• themeEnhanceApp.js - consistent with internal naming.
• app.js
• client.js

We might also consider if we want to add client-side support to the plugins (so plugins can add components, for example, #143 could just be a plugin).

[yyx990803]

Yeah, changed to enhanceApp.js in b668875

[mdaffin]

It is still index.js on master (and in that commit). It looks like you changed the temp file not the source file in prepare.js on L178 - I have created #226 which corrects this.