This commit introduces the following changes in order to improve the
overall robustness of the vimrc:
- All instances of the `ignorecase`-sensitive `=~` operator have been
  replaced with the more robust `=~#` operator.
- The single autocmd has been wrapped in an augroup.

Additionally, it makes the following stylistic improvements:
- Shorthands have been replaced with full option names where applicable.
- Double-quoted strings not containing any substitutions have been
  replaced by single-quoted strings.