Add support for controlling stunnel log debug level (#22)

Is a no-op if not set by the user, since level `notice` is the
stunnel default. See:
https://www.stunnel.org/static/stunnel.html#debug-LEVEL

Copied from heroku-buildpack-pgbouncer:
https://github.com/heroku/heroku-buildpack-pgbouncer/blob/a0601119857fd9170c7bfc64231c29cac8ea143e/bin/gen-pgbouncer-conf.sh#L32

Author: edmorley

README.md

@@ -63,6 +63,7 @@ to any process in the Procfile to run stunnel alongside that process.
 Some settings are configurable through app config vars at runtime:
 
 - ``STUNNEL_ENABLED``: Default to true, enable or disable stunnel.
+- ``STUNNEL_LOGLEVEL``: Default is `notice`, set to `info` or `debug` for more verbose log output.
 - ``STUNNEL_FORCE_TLS``: Default is unset. Set this var, to force TLSv1 on cedar-10.
 
 ### Multiple Redis Instances

bin/stunnel-conf.sh

@@ -24,6 +24,7 @@ options = NO_SSLv3
 TIMEOUTidle = 86400
 ${STUNNEL_FORCE_SSL_VERSION}
 ciphers = HIGH:!ADH:!AECDH:!LOW:!EXP:!MD5:!3DES:!SRP:!PSK:@STRENGTH
+debug = ${STUNNEL_LOGLEVEL:-notice}
 EOFEOF
 
 for URL in $URLS