Add gitattributes file

With two configurations:

1. `* text=auto`: normalizes line endings for all files in the repo
2. `FILE -diff`: hides the diff of these files by default. I configured this
   for lib/main.js and package-lock.json as they are not (intended to be)
   human readable nor edited by humans. Hence, seeing their diff is mostly
   useless.

.gitattributes

@@ -0,0 +1,5 @@
+* text=auto
+
+# don't diff machine generated files
+lib/main.js -diff
+package-lock.json -diff