Show cursor TODO instead of nothing

[danymat]

The idea is basically to show a description for where we want to jump.
Ideally, this should be done with anti conceal (neovim/neovim#9496), but as it's progressing too slowly, we'll have to write to the buffer for the moment.

_Originally posted by @axieax in #34 (comment)

[danymat]

For reference: L3MON4D3/LuaSnip#290

This issue basically exposes the logic of visual selection in insert mode + a luasnip example for creating a snippet in particular position in file.