Invoke NPM only if the global installation dir is in the home directory (fix #21)

src/npm.rs

@@ -0,0 +1,32 @@
+use super::Check;
+use failure;
+use std::path::PathBuf;
+use std::process::Command;
+
+pub struct NPM {
+    command: PathBuf,
+}
+
+impl NPM {
+    pub fn new(command: PathBuf) -> Self {
+        Self { command }
+    }
+
+    pub fn root(&self) -> Result<PathBuf, failure::Error> {
+        let output = Command::new(&self.command).args(&["root", "-g"]).output()?;
+
+        output.status.check()?;
+
+        Ok(PathBuf::from(&String::from_utf8(output.stdout)?))
+    }
+
+    pub fn upgrade(&self) -> Result<(), failure::Error> {
+        Command::new(&self.command)
+            .args(&["update", "-g"])
+            .spawn()?
+            .wait()?
+            .check()?;
+
+        Ok(())
+    }
+}