prevent package-lock diffs (#232)

add package-lock to gitattribute as a binary file, sparing us the extremely verbose diffs.

Author: mwcz

.gitattributes

@@ -2,3 +2,4 @@ elements/*/*.umd.js     -diff -merge
 elements/*/*.umd.js.map -diff -merge
 themes/*/*.umd.js       -diff -merge
 themes/*/*.umd.js.map   -diff -merge
+package-lock.json       -diff -merge