Update build tool's and theme's dependencies

[w568w]

This pull request will:

1. use gomarkdown/markdown as the markdown render, which is an active fork of the dead blackfriday;
   • also, TOC support is on the way for 请问 Ink 用的 blackfriday v1.5 可否升级为 v2 #94, 为毛[TOC]不能用…… #65 and so on;
   • p.s.♻︎: InkProject's fork of blackfriday will be unused after this PR.
2. upgrade dependency versions of default theme, since they are too ancient to build.🏛︎
   • note🚧: precss has been deprecated and should be dropped ASAP.
3. fix some go lint warnings.

[imeoer]

@w568w 大概看了下，看起来很好 👍