Add basic rpc scripts to publish and subscribe (#140)

* Add basic rpc scripts to publish and subscribe

* Fix publish topic and payload script

Also change parameter name in waku relay due to weird shadowing of log
topic:

DBG 2020-09-09 12:07:54+08:00 waku_publish                               tid=8795 file=wakurpc.nim:30 topic=waku payload=@[]
DBG 2020-09-09 12:07:54+08:00 publish                                    tid=8795 file=waku_relay.nim:65 topic=WakuRelay

Above should show topic=waku but it gets topic=WakuRelay from log scope
for some reason.

* Scripts take arguments

* Add basic nangang tutorial

* Update docs/tutorial/nangang.md

Co-authored-by: Kim De Mey <kim.demey@gmail.com>

* Update docs/tutorial/nangang.md

Co-authored-by: Kim De Mey <kim.demey@gmail.com>

* meh

* ENsure subscribe call succeeds

Co-authored-by: Kim De Mey <kim.demey@gmail.com>

docs/tutorial/nangang.md

@@ -0,0 +1,36 @@
+# Nangang Test
+
+Nangang is the first internal testnet. See
+https://github.com/vacp2p/research/issues/43 for more.
+
+## How to
+
+Build:
+
+```
+# make wakunode2 is run as part of scripts2 target
+make scripts2
+```
+
+Run two nodes and connect them:
+
+```
+# Starts listening on 60000 with RPC server on 8545.
+# Note the "listening on address" in logs.
+./build/wakunode2 --ports-shift:0
+
+# Run another node with staticnode argument
+./build/wakunode2 --ports-shift:1 --staticnode:/ip4/0.0.0.0/tcp/60000/p2p/16Uiu2HAmF4tuht6fmna6uDqoSMgFqhUrdaVR6VQRyGr6sCpfS2jp
+```
+
+You should see your nodes connecting.
+
+Do basic RPC calls:
+
+```
+./build/rpc_subscribe 8545
+./build/rpc_subscribe 8546
+./build/rpc_publish 8545 # enter your message in STDIN
+```
+
+You should see other node receive something.