Allow configuring nix-shell integration using zstyle (#613)

pure.zsh

@@ -225,8 +225,10 @@ prompt_pure_precmd() {
 	# Nix package manager integration. If used from within 'nix shell' - shell name is shown like so:
 	# ~/Projects/flake-utils-plus master
 	# flake-utils-plus ❯
-	if [[ -n $IN_NIX_SHELL ]]; then
-		psvar[12]="${name:-nix-shell}"
+	if zstyle -T ":prompt:pure:environment:nix-shell" show; then
+		if [[ -n $IN_NIX_SHELL ]]; then
+			psvar[12]="${name:-nix-shell}"
+		fi
 	fi
 
 	# Make sure VIM prompt is reset.

readme.md

@@ -132,6 +132,10 @@ You can set Pure to only `git fetch` the upstream branch of the current local br
 
 `zstyle :prompt:pure:git:fetch only_upstream yes`
 
+`nix-shell` integration adds the shell name to the prompt when used from within a nix shell. It is enabled by default, you can disable it with:
+
+`zstyle :prompt:pure:environment:nix-shell show no`
+
 ## Colors
 
 As explained in ZSH's [manual](http://zsh.sourceforge.net/Doc/Release/Zsh-Line-Editor.html#Character-Highlighting), color values can be: