vimrc: Use seoul256 color theme

Add mapping to toggle between dark and light
vimrc: Colorscheme: default to dark, unless already set

Prevents light colorscheme from switching to dark when sourcing vimrc
vimrc: Define functions at top of file

Ensures that functions are defined before use
vimrc: Comment out unneeded GitGutter styling

This is already taken care of by the Seoul256 theme
vimrc: Add mapping to fix indentation on code block

(actually just a paragraph)