Force JS files to checkout as LF (#392)

Currently if you checkout the code on Windows the JS files will have CRLF and will automatically fail in ESLint when running 'npm test'

.gitattributes

@@ -1,4 +1,5 @@
 * text=auto
 *.ai binary
+*.js text eol=lf
 readme.md merge=union
 extension/content.css merge=union