Add: Add setup for markdown mode.

Changes: Add setup for markdown mode.
* Enable pandoc-mode in markdown-mode.
Mod: Modify notes in README.
Add: Add setup for latex.

Changes: Add setup for latex.
* Install zotelo by prelude-ensure-module-deps.
* Setup zotelo.
* Setup reftex.
Mod: Install pandoc-mode with prelude-ensure-module-deps.