Fix tab indent for popup

[cocopon]

Thank you for the great plugin! I'm using your plugin with Vim 8.2 and its popup feature:

let g:matchup_matchparen_offscreen = {
  \   'method': 'popup',
  \ }

My environment:

• macOS 10.15.2
• Vim 8.2 (patch 1-319)

I prefer tab indent and sometimes popup window renders it as different width like this:

HTML file for reproducing the behavior (please try it with the option :set tabstop=2):
https://gist.github.com/cocopon/3de4fe7e43a5713ac473ae18688a10fd

I think Vim popup window maybe doesn't follow tabstop option of the current buffer. This pull request will fix the problem by replacing tab characters with spaces.

Neovim floating window works fine so I fixed only the code for Vim popup window.

[andymass]

Thanks for the contribution! (and sorry for the delay, thought I merged this already..)

I think we should make the popup a view of the current buffer (like in the nvim implementation) instead of pure text, then there would be no need to worry about line numbers and tabs.