* README.md: Mention zstandard
* configure.ac: Check for libzstd
* docs/wget2.md: Document new --compression type
* include/wget/wget.h: Add wget_content_encoding_zstd
* libwget/decompressor.c: Implement the zstd decompressor
* libwget/http_parse.c (wget_http_parse_content_encoding): Add zstd
* src/options.c (print_version): Add +/-zstd for --version,
  (parse_compression): Check for wget_content_encoding_zstd,
  (options): Add zstd to help text
* src/wget.c (http_create_request): Add zstd to Accept-Encoding:
* tests/test-compression.c: Add test for zstd