Please support an XDG location for .agignore like Git does for its .gitignore.

Benefits

• No need for symlinks when maintaining dotfiles
• No need for an alias
• XDG conformity

Logic

Use first found in this order:

1. Check if "${HOME}/.agignore" exists
2. If not, check if "${XDG_CONFIG_HOME:=$HOME/.config}/ag/ignore" exists.