Add `coverage` directory, rename coverage heading

Node.gitignore

@@ -16,11 +16,14 @@ results
 *.out
 *.gz
 
-# JS-specific patterns
+# Code coverage patterns
 
-## Directory for files generated by jscoverage/JSCover
+## Directory for coverage-hooked libs generated by jscoverage/JSCover
 lib-cov
 
+## Coverage directory used by tools like istanbul
+coverage
+
 # Node-specific patterns
 
 ## Compiled binary addons (http://nodejs.org/api/addons.html)