config: allow web-proxy support

Now any of the following env variables may be exported before using
cmk with any http/https proxy:

export http_proxy='http://user:password@server:3128'
export https_proxy='http://user:password@server:3128'
export HTTP_PROXY='http://user:password@server:3128'
export HTTPS_PROXY='http://user:password@server:3128'

Or, run cmk like following:

HTTP_PROXY="http://IP:port" cmk

Fixes #49

Signed-off-by: Rohit Yadav <rohit.yadav@shapeblue.com>

config/config.go

diff --git a/config/config.go b/config/config.go
index 04158cb..664d99b 100644
--- a/config/config.go
+++ b/config/config.go
@@ -153,6 +153,7 @@
 	client := &http.Client{
 		Jar: jar,
 		Transport: &http.Transport{
+			Proxy:           http.ProxyFromEnvironment,
 			TLSClientConfig: &tls.Config{InsecureSkipVerify: !cfg.Core.VerifyCert},
 		},
 	}