extract date field from folder name, instead of subfile

[mohkale]

I'm not a big fan of specifying the date twice, eg:

- 2019-06-19-welcome/
  - 2019-06-19-welcome-to-jekyll.md

I know the parent folder doesn't need to have the date, but if it doesn't then it doesn't match the contents of the sorrounding posts. I'd prefer a directory format like this.

- 2019-06-19-welcome-to-jekyll/
  - index.md

[mohkale]

Note: This is low priority.

[astrosticks]

Ditto on this. I'd love to only have to name my post once.

I think it should look for 2019-06-19-welcome-to-jekyll/ first, in case someone is migrating from a regular jekyll blog and doesn't want to rename their files, and if that file doesn't exist, look for index.md.