Allow theme config to be overridden by site config

[nielinjie]

I agree and want to create new issue

• Yes, I was on Hexo Docs page, especially on Templates, Variables, Helpers and Troubleshooting.
• Yes, I was on NexT Documentation Site.
• And yes, I already searched for current issues and this is not help to me.

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Expected behavior

Theme config can be override by site config, following hexo way -
hexojs/hexo#757

Actual behavior

Using _data file override.

Steps to reproduce the behavior

1. N/A

• Link to demo site with this issue: N/A
• Link(s) to source code or any usefull link(s): N/A

Hexo Information

hexo: 3.4.1
hexo-cli: 1.0.4
os: Darwin 17.2.0 darwin x64
http_parser: 2.7.0
node: 8.6.0
v8: 6.0.287.53
uv: 1.14.1
zlib: 1.2.11
ares: 1.10.1-DEV
modules: 57
nghttp2: 1.25.0
openssl: 1.0.2l
icu: 59.1
unicode: 9.0
cldr: 31.0.1
tz: 2017b

NexT Information

NexT Version:

[x] Latest Master branch.
[] Latest Release version.
[] Old version - 

NexT Scheme:

[x] All schemes
[] Muse
[] Mist
[] Pisces
[] Gemini

Other Information

[ivan-nginx]

Fixed in NexT v6 here. Docs here.