message: feat(config): Change future destination to _site
  This is inspired by Jekyll.
  The previous value of `.` causes odd behavior of trying to clean the
  directory.
  The new value works well with cobalt's default way of hiding things.
diff: @@ -131,7 +131,7 @@ impl Default for ConfigBuilder {
  ConfigBuilder {
  root: path::PathBuf::new(),
  source: "./".to_owned(),
  - destination: "./".to_owned(),
  + destination: "./_site".to_owned(),
  abs_dest: None,
  include_drafts: false,
  default: frontmatter::FrontmatterBuilder::new()
  