Async run

Sometimes it can take a little time to execute this extension. For instance if I automatically triggers the coverage highlighting at buffer opening with `autocmd! BufWritePost,BufEnter *.py HighlightCoverageForAll` it can take more than 1s for the buffer to display.

I understood that since vim 8 or neovim, commands can be ran asynchronously. Could it be possible to run this extension asynchronously?

Start from the HighlightCoverageForAll entry point invoked by the BufWritePost and BufEnter autocmds, and trace how it currently performs coverage highlighting. Done should mean the extension can run asynchronously without delaying buffer display when highlighting is triggered at buffer opening.

Written by the indexing model from the issue text.