EPUB: fix subjects

Also update README

README.md

@@ -95,7 +95,7 @@ Processors for each book format return an object that implements the following i
     - `.href`: a string representing the destination of the item. Does not have to be a valid URL.
     - `.subitems`: a array that contains TOC items
 - `.pageList`: same as the TOC, but for the [page list](https://www.w3.org/publishing/epub32/epub-packages.html#sec-nav-pagelist).
-- `.metadata`: an object representing the metadata of the book.
+- `.metadata`: an object representing the metadata of the book. Currently, it follows more or less the metadata schema of [Readium's webpub manifest](https://github.com/readium/webpub-manifest).
 - `.rendition`: an object that contains properties that correspond to the [rendition properties](https://www.w3.org/publishing/epub32/epub-packages.html#sec-package-metadata-rendering) in EPUB. If `.layout` is `"pre-paginated"`, the book is rendered with the fixed layout renderer.
 - `.resolveHref(href)`: given an href string, returns an object representing the destination referenced by the href, which has the following properties:
     - `.index`: the index of the referenced section in the `.section` array

epub.js

@@ -134,7 +134,13 @@ const METADATA = [
     { name: 'rights', ...LANGS, props: [ALTS] },
     { name: 'date' },
     { name: 'dcterms:modified', type: 'meta' },
-    { name: 'subject', many: true, ...LANGS, props: ['term', 'authority', ALTS] },
+    {
+        name: 'subject', many: true, ...LANGS, props: ['term', 'authority', ALTS],
+        setLegacyAttrs: (obj, el) => {
+            obj.term ??= el.getAttributeNS(NS.OPF, 'term')
+            obj.authority ??= el.getAttributeNS(NS.OPF, 'authority')
+        },
+    },
     { name: 'source', many: true },
     {
         name: 'belongs-to-collection', type: 'meta', many: true, ...LANGS,
@@ -888,8 +894,9 @@ ${doc.querySelector('parsererror').innerText}`)
             published: metadata?.date,
             modified: metadata?.dctermsModified,
             subject: metadata?.subject
-                ?.filter(({ value, code }) => value || code)
+                ?.filter(({ value, term }) => value || term)
-                ?.map(({ value, code, scheme }) => ({ name: value, code, scheme })),
+                ?.map(({ value, term, authority }) =>
+                    ({ name: value, code: term, scheme: authority })),
             rights: metadata?.rights?.value,
         }
         const relators = {