Log Message:
(devel/R-xfun) Updated 0.37 to 0.39

# CHANGES IN xfun VERSION 0.39

- Fixed a bug that `protect_math()` fails to protect the starting `$$`
  that has leading white spaces.

- Added a function `strip_html()` to remove HTML tags and comments
  from text.

- The function `alnum_id()` will remove HTML tags and comments from
  text (using `strip_html()`) before converting it to an ID string.

- Added a function `env_option()` to retrieve an option value from
  `options()`. If the option does not exist there, check the
  environment variables. This provides a way for users to set an
  option via either `options()` or an environment variable.

# CHANGES IN xfun VERSION 0.38

- Added an object `download_cache`, which is a list of methods to
  download a URL, cache the result, retrieve the result from the
  cache, and clear the cache.

- Added an argument `default` to `url_filename()` to provide a default
  filename when it cannot be determined from the URL.

- Added a function `yaml_load()` to read YAML data when the **yaml**
  package is not available. It only supports a limited number of data
  types and is supposed to be used as a fallback method. See the help
  page `?xfun::yaml_load` for details.

- Added a function `yaml_body()` to split a document into YAML
  metadata and the body.

- `is_arm64()` also supports Linux now (thanks, @eitsupi, #74).

- `is_blank()` returns a logical vector of the same length as the
  input vector now, indicating if each element of the input is
  blank. Previously it returns a logical scalar indicating whether
  *all* elements are blank. If you want the old behavior, you can use
  `all(is_blank())`.