chore(package): delete jscs & use eslint

[yoshinorin]

jscs has already merged with eslint.

fix ESLint error

I fix 6 eslint error after migrate to eslint.

$ yarn eslint
$ eslint .

./hexo-generator-feed/lib/generator.js
   7:24  error  "hexo" is not published                                            node/no-unpublished-require
  14:22  error  Unexpected control character(s) in regular expression: \x00, \x1f  no-control-regex

./hexo-generator-feed/test/index.js
   16:22  error  Unexpected control character(s) in regular expression: \x00, \x1f            no-control-regex
   36:3   error  Unnecessary parentheses around expression                                    no-extra-parens
   39:3   error  Combine this with the previous 'var' statement with uninitialized variables  one-var
  116:35  error  Unnecessary escape character: \:                                             no-useless-escape

✖ 6 problems (6 errors, 0 warnings)
  2 errors and 0 warnings potentially fixable with the `--fix` option.

[coveralls]

Coverage remained the same at 100.0% when pulling b397303 on YoshinoriN:delete-jscs-and-use-eslint into b67e9de on hexojs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b397303 on YoshinoriN:delete-jscs-and-use-eslint into b67e9de on hexojs:master.

[coveralls]

Coverage remained the same at 100.0% when pulling b397303 on YoshinoriN:delete-jscs-and-use-eslint into b67e9de on hexojs:master.