Test all features

Add cases for attribute highlight on and off, block syntax by lang
and emmet-vim group names, OnChangeVueSyntax(), matchit, .wpy,
initial_indent as syntax and tag.syntax, and syntax after reloading.

Also check indenting a file after removing all indent with
g:test_flatten, which fails before the end tag fix as closing tags at
column 0 were taken as block end tags.

test/cases/initial-indent.vim

@@ -0,0 +1,10 @@
+" initial_indent as `syntax` and `tag.syntax`
+let g:vim_vue_plugin_config = {
+      \'syntax': {
+      \   'script': ['javascript'],
+      \   'style': ['css', 'scss'],
+      \   'i18n': ['json', 'yaml'],
+      \},
+      \'initial_indent': ['scss', 'i18n.json'],
+      \}
+let g:test_flatten = 1