Reduce lodash dependent code

[segayuu]

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• Add test cases for the changes.
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[coveralls]

Coverage decreased (-0.003%) to 97.27% when pulling da3249c on segayuu:Reduce-lodash-dependent-code into d9196e3 on hexojs:master.

[coveralls]

Coverage decreased (-0.003%) to 97.27% when pulling da3249c on segayuu:Reduce-lodash-dependent-code into d9196e3 on hexojs:master.

[yoshinorin]

I think lodash/isDate maybe can replace instanceof Date.

[segayuu]

that's right!
In addition, lodash.isDate() returns the result if util.types.isDate() exists, if one exists.
Although it may be corrected, I think that the necessity to thoroughly perform the contents of this PR is thinner than other libraries such as warehouse.
That's because hexo exports the lodash namespace and it's a big size as a library.

[yoshinorin]

@segayuu
I tried hexo server and hexo generate using by your branch source code in my local environment.
Like this.

  "dependencies": {
    "hexo": "segayuu/hexo#Reduce-lodash-dependent-code"
    "hexo-generator-archive": "0.1.5",
    "hexo-generator-category": "0.1.3",
    ...
    "hexo-server": "0.3.3"
  }

It seems works well. I think this PR is no problem.
I will merge this PR if lodash/isDate question will be clear.

[yoshinorin]

OK, Merged !

[segayuu]

thank @yoshinorin !