* feat: support toml config

* remove config.toml

* fix: proper iteration over meta tags

Until now, only `README.md` files got treated as a directory index.
This commit also allows `index.md` to be used as index files.

If vuepress is run in the root of a node package
(eg. `vuepress build .`), it would include all markdown files from
the `node_modules` folder, which breaks the build process.