re-added endpoint parsing login (originally removed and replced with …

[DustinKLo]

…a region flag)

made -region flag optional, if not supplied will go through the endpoint parsing logic instead
edited README.md description for region flag
added "assume" description to --help in README.md

[DustinKLo]

still works 👍

$ docker run -p 9200:9200 aws_es_proxy:latest -endpoint https://vpce-#####-####.ce-svc-#####.us-####-#.vpce.amazonaws.com -listen 0.0.0.0:9200 -region us-west-2 -insecure -verbose -pretty
time="2020-15-11 20:23:15" level=info msg="Listening on 0.0.0.0:9200...\n"
time="2020-15-11 20:23:18" level=info msg="Using default credentials"
time="2020-15-11 20:23:18" level=info msg="Generated fresh AWS Credentials object"

========================
2020/11/15 20:23:21
Remote Address:  172.17.0.1:40010
Request URI:  /
Method:  GET
Status:  200
Took: 3.285s
Body:

$ curl localhost:9200
{
  "name" : "6a085e466b8afaf71ef350492e7e03cd",
  "cluster_name" : "271039147104:nisar-dev",
  "cluster_uuid" : "t_nlyhQDRNqvEu5GHBC8ng",
  "version" : {
    "number" : "7.1.1",
    "build_flavor" : "oss",
    "build_type" : "tar",
    "build_hash" : "7a013de",
    "build_date" : "2019-09-05T07:25:23.525600Z",
    "build_snapshot" : false,
    "lucene_version" : "8.0.0",
    "minimum_wire_compatibility_version" : "6.8.0",
    "minimum_index_compatibility_version" : "6.0.0-beta1"
  },
  "tagline" : "You Know, for Search"
}