Allow using nvim for processing helptags and more

[comfortablynick]

Neovim-only compatibility

I only have Neovim installed on my system, and I was getting a panic with "something went wrong" as the message after plugin installs. This PR allows nvim to be called at the end of the install process if vim returns NotFound. With this change, things seem to work fine (so far) with Neovim.

Fix clippy warnings

• Replace deprecated env::home_dir() with equivalent from dirs crate
• Use character instead of single character string
• Use iter() instead of into_iter() in one place

There are clippy warnings about redundant closures, but that may be a bug.

[maralla]

Thanks!