Support deleting/changing custom surrounds

[tomtomjhj]

Fixes #199, #210, #318, #346

I tested on these custom surrounds, and it seems to work pretty well.

let g:surround_{char2nr('c')} = "/* \r */"
let g:surround_{char2nr('m')} = "(* \r *)"
let g:surround_{char2nr('M')} = "(** \r *)"
let g:surround_{char2nr('b')} = "**\r**"
let g:surround_{char2nr('l')} = "\\begin{\1environment: \1}\r\\end{\1\r}.*\r\1}"
let g:surround_{char2nr('d')} = "<div\1id: \r..*\r id=\"&\"\1>\r</div>"

[isovector]

This PR is amazing, thanks. It's an important enough feature that I'm using your fork in place of tpope/vim-surround. Definitely worth a merge, @tpope !

[ggVGc]

I also just ran into this, and found this solution.
Any reason to not bring it in @tpope?