Add plug#helptags()

test/run

@@ -30,6 +30,10 @@ EOF
 
 make_dirs xxx/
 make_dirs xxx/after
+mkdir xxx/doc
+cat > xxx/doc/xxx.txt << DOC
+hello *xxx*
+DOC
 
 cat > /tmp/mini-vimrc << VIMRC
 set rtp+=vader.vim

test/workflow.vader

@@ -761,6 +761,16 @@ Execute (Filetype-based on-demand loading):
   setf xxx
   AssertEqual ['/ftdetect', 'after/ftdetect', '/plugin', 'after/plugin', '/ftplugin', 'after/ftplugin', '/indent', 'after/indent', '/syntax', 'after/syntax'], g:xxx
 
+**********************************************************************
+~ plug#helptags()
+**********************************************************************
+
+Execute (plug#helptags):
+  silent! call delete(expand('$PWD/xxx/doc/tags'))
+  Assert !filereadable(expand('$PWD/xxx/doc/tags'))
+  AssertEqual 1, plug#helptags()
+  Assert filereadable(expand('$PWD/xxx/doc/tags'))
+
 Before:
 Execute (Cleanup):
   silent! call system('rm -rf '.temp_plugged)