Use treesitter for folding

[jdhoffa]

This is a super small feature that:

• Enables expr based folding in neovim based on treesitter parsers
• Adds autocmd to ensure code is unfolded when new file or buffer is opened

Feel free to close the PR if you don't think you would use this, I'm just reviewing how my fork has diverged from your project, and trying to extract things you might find useful :-)

[jdhoffa]

There was some weird commit signing going on here, since I had your .gitconfig checked out when I was committing to this branch. Closed in favour of #5