message: feat(front): permalink aliases
  So far this only adds `path` which is also the default.  Also, this is
  not yet accessible until we switch formats.
  This is a step towards
diff: use std::path;
  +use std::collections::HashMap;
  +
  use liquid;
  use error::Result;
  use datetime;
  use slug;
  +const PATH_ALIAS: &'static str = "/:path/:filename:output_ext";
  +lazy_static!{
  + static ref PERMALINK_ALIASES: HashMap<&'static str, &'static str> = [
  + ("path", PATH_ALIAS),
  + ].iter().map(|&(k, v)| (k, v)).collect();
  +}
  +
  #[derive(Debug, Eq, PartialEq, Hash, Copy, Clone)]
  #[derive(Serialize, Deserialize)]
  #[serde(deny_unknown_fields)]
  @@ -300,11 +309,16 @@ impl FrontmatterBuilder {
  let is_post = is_post.unwrap_or(false);
  - let permalink = permalink.unwrap_or_else(|| {
  - let default_permalink = "/:path/:filename:output_ext";
  -
  - default_permalink.to_owned()
  - });
  + let permalink = permalink.unwrap_or_else(|| PATH_ALIAS.to_owned());
  + let permalink = if !permalink.starts_with('/') {
  + let resolved =
  + *PERMALINK_ALIASES
  + .get(permalink.as_str())
  + .ok_or_else(|| format!("Unsupported permalink alias '{}'", permalink))?;
  + resolved.to_owned()
  + } else {
  + permalink
  + };
  let fm = Frontmatter {
  permalink: permalink,
  