REUSE: Add Copyright for myhtmlex

git ls-tree --name-only -r 18593ba4109c6f3eecf63627ccae150bf911f091 | while read name; do test -e "$name" && reuse addheader --year 2017-2019 --copyright 'myhtmlex authors https://github.com/Overbryd/myhtmlex' --license 'LGPL-2.1-only' "$name"; done
reuse addheader --year 2017-2019 --copyright 'myhtmlex authors https://github.com/Overbryd/myhtmlex' --license 'LGPL-2.1-only' lib/fast_html.ex
reuse addheader --year 2017-2019 --copyright 'myhtmlex authors https://github.com/Overbryd/myhtmlex' --license 'LGPL-2.1-only' test/fast_html_test.exs