chore(deps): support hexo 4.2.0 (#22)

[yoshinorin]

Description

fixes: #22

In hexo 4.2.0 Locals class use hexo-util. Please see followings:

hexojs/hexo#3996
hexojs/hexo@c6e6e6b#diff-8706ed85bcc456bc4bd99698f3cb949dR7

Fix

hexo.locals.cache to hexo.locals.cache.cache

Others

• I add engines option to package.json. Because current hexo support +8.10.0
• I test only related post feature.

Thank you :)

[coveralls]

Coverage remained the same at 78.947% when pulling f422b7c on YoshinoriN:hexo4.2.0-support into 5f782be on tea3:master.

[yoshinorin]

At first glance, on my local machine works well.
But, the performance problem occurred.

Browser reload is very slow when post file change...

[SukkaW]

@yoshinorin Locals.cache.cache should be considered a private API. We could add a dump() in Hexo 4.3.0, so that we will have a consistent API.

According to Hexo API's documents, there is already a method called toObject(). The plugin should use Locals.toObject() for consistency. And cache for toObject() should be handled by plugin itself.

[yoshinorin]

@SukkaW
Thank you :) I will check it later.