fix(url_for): some improvement for `url_for`

[dimaslanjaka]

chore: some improvement for url_for

1. make options as optional with default value as empty object
   • sample undefined options - https://github.com/next-theme/hexo-theme-next/blob/acca4e83747b168e90815945f58f982b1d14eac5/layout/_layout.njk#L10
   • sample usage in plugin - https://github.com/dimaslanjaka/hexo-shortcodes/blob/c099f51c412d794136ff9064bf0d182ae17a1573/src/gist.ts#L42-L49
2. add JSDoc
   • add sample usage inside plugin/scripts
3. assign options fallback to an empty object
   • sometimes i saw syntax using url_for the options given as null, prevent useless error give options fallback to an empty object

[coveralls]

coverage: 96.793% (+0.05%) from 96.74% when pulling b045658e592ac302deb58328e10bd43ab675f7ec on dimaslanjaka:fix_url_for into 460621e4333eb6d810cd7567739473e3f8bda51d on hexojs:master.

[dimaslanjaka]

@SukkaW review pls