Replace language editor buttons with paper-button

The font family for the options page has also been changed so the glyphs
used in the buttons appear better in Mac (and hopefully in other OSs
too).

Also, the node_modules is now excluded from the source-map-loader rule
in order to reduce the extension tarball size.

Bug: translateselectedtext:12
Change-Id: I399987a87c8d2de147d46ce1cf89f36b9e2e4a9d
diff --git a/package.json b/package.json
index 4ce1444..74a3012 100644
--- a/package.json
+++ b/package.json
@@ -25,6 +25,7 @@
   "license": "MIT",
   "private": true,
   "dependencies": {
+    "@polymer/paper-button": "^3.0.1",
     "chrome-types": "^0.1.113",
     "clean-terminal-webpack-plugin": "^3.0.0",
     "clean-webpack-plugin": "^4.0.0",