Add kill switch mechanism

This code implements the kill switch mechanism in the extension. This is
explained in //src/killSwitch/README.md and in the design doc:
https://docs.google.com/document/d/1O5YV6_WcxwrUyz-lwHOSTfZ3oyIFWj2EQee0VuKkhaA/edit.

Bug: twpowertools:64

Change-Id: Ia993c78035bba7038aafd53d156f20954217e86f
diff --git a/src/static/options/options.css b/src/static/options/options.css
index 4ab69b3..a3cd7ab 100644
--- a/src/static/options/options.css
+++ b/src/static/options/options.css
@@ -24,11 +24,15 @@
   cursor: pointer;
 }
 
-.option {
+.option, .kill-switch-text {
   margin: 4px 0;
   line-height: 1.8em;
 }
 
+.kill-switch-text {
+  color: red;
+}
+
 .help {
   cursor: help;
   border-bottom: dashed 1px gray;