v0.3.1

What's Changed

• Dropped dependency on envrc package. Instead we get the shell environment from
  nix-shell using direnv's dump command. No more out-of-tree directory with .envrc.
  This gives us a noticeable performance win.

• Removed org-nix-shell-get-direnv-path, org-nix-shell-envrc-format and
  org-nix-shell-src-block-name. Instead we tangle nix shells directly to "/tmp/"

• Linter and documentation by @AntonHakansson in #4

Full Changelog: v0.2.0...v0.3.1