message: Add period to allowed feature name characters.
diff: @@ -460,11 +460,11 @@ fn validate_feature_name(config: &Config, pkg_id: PackageId, name: &str) -> Carg
  }
  }
  for ch in chars {
  - if !(unicode_xid::UnicodeXID::is_xid_continue(ch) || ch == '-' || ch == '+') {
  + if !(unicode_xid::UnicodeXID::is_xid_continue(ch) || ch == '-' || ch == '+' || ch == '.') {
  config.shell().warn(&format!(
  "invalid character `{}` in feature `{}` in package {}, \
  - characters must be Unicode XID characters or `+` \
  - (numbers, `+`, `-`, `_`, or most letters)\n\
  + characters must be Unicode XID characters, `+`, or `.` \
  + (numbers, `+`, `-`, `_`, `.`, or most letters)\n\
  {}",
  ch, name, pkg_id, FUTURE
  ))?;
  