Added MOONSTREAM_HUMBUG_TOKEN environment variable

The ethereum trending addresses crawler will be the first one to report
data through a Humbug journal. This requires us to accept a Humbug token
as configuration.

crawlers/sample.env

@@ -2,3 +2,4 @@
 export MOONSTREAM_IPC_PATH=null
 export MOONSTREAM_CRAWL_WORKERS=4
 export MOONSTREAM_DB_URI="postgresql://<username>:<password>@<db_host>:<db_port>/<db_name>"
+export MOONSTREAM_HUMBUG_TOKEN="<Token for crawlers store data via Humbug>"