Add command to show single bookmark details

`/bookmark list` lists all bookmarks with its details.
`/bookmark list <jid>` shows the details of a single bookmark.

Implement https://github.com/profanity-im/profanity/issues/1558

src/xmpp/bookmark.c

@@ -225,6 +225,13 @@ bookmark_remove(const char* jid)
     return TRUE;
 }
 
+Bookmark*
+bookmark_get_by_jid(const char* jid)
+{
+    Bookmark* bookmark = g_hash_table_lookup(bookmarks, jid);
+    return bookmark;
+}
+
 GList*
 bookmark_get_list(void)
 {

src/xmpp/xmpp.h

@@ -274,6 +274,7 @@ gboolean bookmark_update(const char* jid, const char* nick, const char* password
 gboolean bookmark_remove(const char* jid);
 gboolean bookmark_join(const char* jid);
 GList* bookmark_get_list(void);
+Bookmark* bookmark_get_by_jid(const char* jid);
 char* bookmark_find(const char* const search_str, gboolean previous, void* context);
 void bookmark_autocomplete_reset(void);
 gboolean bookmark_exists(const char* const room);