feat(markdown): Add filetype config for markdown

markdown files will now by default:
- Have spell on
- Have a textwidth of 75
- Highlight columns 75 & 80

ftplugin/markdown.lua

@@ -0,0 +1,3 @@
+vim.wo.spell = true
+vim.bo.textwidth = 75
+vim.wo.colorcolumn = "75,80"