Conceal rules missing in a bunch of colorschemes

[fmoralesc]

At vim-pandoc-syntax, we've found that many colorschemes don't have explicit rules for the Conceal group, which creates this sort of issue (see the grey background in the first half of the text):

Checking on the contents of this repo, only a few colorschemes provide correct rules for Conceal: https://github.com/flazz/vim-colorschemes/search?q=Conceal&source=cc

I'm not sure this is the right place to raise this issue, but I thought it would be good to let you know.

We've found adding

hi link Conceal Operator

to the colorschemes works fine in our use-case, but a different rule might be needed in others.

[flazz]

PR?