Add login/logout and store token to config file

[Cody A. Ray (codyaray)]

@confluentinc/caas

$ go run main.go login --url https://stag.cpdev.cloud
2018-05-02T12:52:48.012-0500 [DEBUG] plugin: starting plugin: path=/bin/sh args="[sh -c /Users/cody/code/go/bin/confluent-connect-plugin]"
2018-05-02T12:52:48.014-0500 [DEBUG] plugin: waiting for RPC address: path=/bin/sh
2018-05-02T12:52:48.028-0500 [DEBUG] plugin.sh: plugin address: address=/var/folders/81/kbvhd83n2sj12dy2xnxjc0140000gp/T/plugin575483489 network=unix timestamp=2018-05-02T12:52:48.028-0500
Enter your Confluent Cloud credentials:
Email: cody@confluent.io
Password: 
Logged in as cody@confluent.io

$ cat ~/.confluent/config.hcl
"auth_token" = "eyJhbGciOiJIUzI1NiIsInR5cCI6Ik..."

$ go run main.go logout
2018-05-02T12:53:06.544-0500 [DEBUG] plugin: starting plugin: path=/bin/sh args="[sh -c /Users/cody/code/go/bin/confluent-connect-plugin]"
2018-05-02T12:53:06.546-0500 [DEBUG] plugin: waiting for RPC address: path=/bin/sh
2018-05-02T12:53:06.557-0500 [DEBUG] plugin.sh: plugin address: address=/var/folders/81/kbvhd83n2sj12dy2xnxjc0140000gp/T/plugin083074154 network=unix timestamp=2018-05-02T12:53:06.557-0500
You are now logged out

$ cat ~/.confluent/config.hcl
"auth_token" = ""

Logging still needs some work. Updated logrus to tweak the output a bit. Still need to integrate/silence the go-plugin logging for non-debug usage.