[Feature Request] Support Vim Sessions

[ColinKennedy]

If you use Vim Sessions, e.g. :mksession Session.vim and then nvim -S Session.vim, the terminal buffer is restored but toggleterm.nvim doesn't recognize the restored terminal buffer as a "toggleterm" buffer. So the next :ToggleTerm call creates a second terminal. There's also other issues as well, for example the restored terminal won't have tte same styling as if it were created using :ToggleTerm.

Being able to restore the toggleterm terminal will likely require #528 so I'd consider the two issues to be related.