Corp with other markdown syntax plugins

[tyru]

Current Status

Currently, this plugin only works with tpope/vim-markdown.
This is included in runtime files but currently included one does not corp with markdown-codehl-onthefly.vim (tpope/vim-markdown#92).

Other markdown syntax plugins

• tpope/vim-markdown
  • Avoid E403 error on :syn-include tpope/vim-markdown#92
  • Seems not included in Vim runtime files
• gabrielelana/vim-markdown
• rcmdnk/vim-markdown
• plasticboy/vim-markdown

[tyru]

Updated issue text according to README.md.