添加生成分类主页面支持 Add support to generate category index page

[pmtao]

添加设置参数：enable_index_page
Add config item: enable_index_page

[coveralls]

Coverage decreased (-88.9%) to 11.111% when pulling dc1fd2d on pmtao:master into 80f03fa on hexojs:master.

[coveralls]

Coverage decreased (-88.9%) to 11.111% when pulling dc1fd2d on pmtao:master into 80f03fa on hexojs:master.

[NoahDragon]

Could you please fix the travis-ci errors?

[pmtao]

抱歉，还没用过 travis-ci ，能不能稍微指点下怎么处理，谢谢！

[coveralls]

Coverage decreased (-80.0%) to 20.0% when pulling 733165c on pmtao:master into 80f03fa on hexojs:master.

[pmtao]

我还原到上一版代码，问题还是基本类似的： travis-ci 日志。
而且下面这个问题说明 babel-eslint 在 node 0.12 和 4 的版本上可能就不支持，是不是要考虑支持5以上：Cannot find module 'babel-eslint'

[JLHwung]

@pmtao Could you rebase on the master branch to have travis run test on node_js 6 and 8? Thank you.

[pmtao]

@JLHwung travis 现在有问题：Open-source builds erroring

[NoahDragon]

@JLHwung I think the error is caused by the babel-eslint, this repo doesn't contain it. I will fix it.

[JLHwung]

@NoahDragon The build is fixed due to recent changes on hexo.

@pmtao On your working branch master, could you run

git remote add upstream https://github.com/hexojs/hexo-generator-category.git
git fetch upstream
git rebase upstream/master

and use git push origin --force to see if the test pass again? Thank you.