* Add vimtex plugin
Vimtex provides a full integration for latex files, see: https://github.com/lervag/vimtex.
Zathura is used as default pdf viewer as it allows live reloading.
* Provide the <leader>L shortcut for Latex operations.
* Move vimtex into the extra plugins section
Add a short comment above extra plugins to introduce their functionality
* Only define whcihKey mapping for Vimtex if O.extras is true
Co-authored-by: Christian Chiarulli <chris.machine@pm.me>
