Use src instead of lib for directory structure

[jamos-tay]

Taken from #467 (comment)

As suggested with src/plugins/render-tags.js, let's start using src/ instead of lib/.

• src/ is pretty common.
• libs/ implies libraries' code, rather than this library's code. It's more nested but clearer.

Current directory structure:

lib/
 asset/
 markbind/
 lib/
 markdown-it/
 parser.js
 utils.js
 package-lock.json
 package.json
 template/
 util/
 Page.js
 Site.js
package-lock.json
package.json

Proposed directory structure:

src/
 asset/
 lib/
 markbind/
 src/
 lib/
 markdown-it/
 parser.js
 utils.js
 package-lock.json
 package.json
 template/
 util/
 Page.js
 Site.js
package-lock.json
package.json