fix unexpected error when options of cache-loader contains !

[ulivz]

Summary

When cacheIdentifier contains exclamation mark !, the ! would be treated as the separator between different loaders. which will cause unexpected compilation error.

e.g.

when you have these options set:

{
     cacheIdentifier: 'a!a=1'
}

You would get following error:

Module not found: Error: Can't resolve 'a=1-vue-loader-template"}' in '/Users/haolchen/Documents/__self__/__forked__/vuepress/docs/guide'
 @ ./docs/guide/deploy.md?vue&type=template&id=7ff0bdcd 1:0-324 1:0-324
 @ ./docs/guide/deploy.md
 @ ./lib/app/.temp/routes.js
 @ ./lib/app/app.js
 @ ./lib/app/clientEntry.js
 @ multi ./lib/app/clientEntry.js

This PR is to replace all the !s in cacheIdentifier, and document that cacheDirectory path cannot contain ! (We cannot replace the !s in cacheDirectory).

refs: vuejs/vuepress#532

Updated

As Evan's comment, we just hash the whole cacheIdentifier string.

[yyx990803]

I think it's better to just hash the whole string with hash-sum, since other chars that may have special meanings in inline requests can also cause issues.

[ulivz]

Fixed at 448e5b9