spell: rename "aspell" plugin to "spell" (issue #1299)

The following things have been renamed:

- file aspell.conf -> spell.conf
- options aspell.* -> spell.*
- bar item aspell_dict -> spell_dict
- bar item and local variable aspell_suggest -> spell_suggest
- info aspell_dict -> spell_dict

src/plugins/spell/spell-command.h

@@ -0,0 +1,25 @@
+/*
+ * Copyright (C) 2013-2019 Sébastien Helleu <flashcode@flashtux.org>
+ *
+ * This file is part of WeeChat, the extensible chat client.
+ *
+ * WeeChat is free software; you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation; either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * WeeChat is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ *
+ * You should have received a copy of the GNU General Public License
+ * along with WeeChat.  If not, see <https://www.gnu.org/licenses/>.
+ */
+
+#ifndef WEECHAT_PLUGIN_SPELL_COMMAND_H
+#define WEECHAT_PLUGIN_SPELL_COMMAND_H
+
+extern void spell_command_init ();
+
+#endif /* WEECHAT_PLUGIN_SPELL_COMMAND_H */