fix(zplug): search appropriate paths for zplug folder

Check ZPLUG_HOME environment variable in addition to ~/.zplug. Also consider ZDOTDIR when sourcing
.zshrc.

src/steps/os/unix.rs

@@ -1,20 +1,44 @@
-use crate::error::Error;
+use crate::error::{Error, ErrorKind::*};
 use crate::executor::{CommandExt, RunType};
 use crate::terminal::print_separator;
 use crate::utils::{require, which};
 use directories::BaseDirs;
 use std::path::Path;
 use std::process::Command;
+use std::env;
+
+fn zplug_exists(base_dirs: &BaseDirs) -> bool {
+    let home_exists = match env::var("ZPLUG_HOME") {
+        Ok(ref zplug_home) => Path::new(zplug_home).exists(),
+        Err(_) => false,
+    };
+    let dotdir_exists =  base_dirs.home_dir().join(".zplug").exists();
+    home_exists || dotdir_exists
+}
+
+fn get_zshrc(base_dirs: &BaseDirs) -> Result<String, ()> {
+    let zshrc = match env::var("ZDOTDIR") {
+        Ok(ref zdotdir) => Ok(Path::new(zdotdir).join(".zshrc")),
+        Err(_) => Err(()),
+    };
+    zshrc
+        .unwrap_or(base_dirs.home_dir().join(".zshrc"))
+        .to_str()
+        .map(|s| s.to_owned())
+        .ok_or(())
+}
 
 pub fn run_zplug(base_dirs: &BaseDirs, run_type: RunType) -> Option<(&'static str, bool)> {
     if let Some(zsh) = which("zsh") {
-        if base_dirs.home_dir().join(".zplug").exists() {
+        if zplug_exists(base_dirs) {
             print_separator("zplug");
 
             let success = || -> Result<(), Error> {
+                let zshrc = get_zshrc(base_dirs).map_err(|_| Error::from(SkipStep))?;
+                let cmd = format!("source {} && zplug update", zshrc);
                 run_type
                     .execute(zsh)
-                    .args(&["-c", "source ~/.zshrc && zplug update"])
+                    .args(&["-c", cmd.as_str()])
                     .check_run()?;
                 Ok(())
             }()