#409: Improve endpoint determination code layer

• Implement determine_endpoint() to replace Endpoint()
• It handles custom, default g1 and gtest endpoints
• Read custom endpoints from a regex
• Handle host/ipv4 mixup fix with ipaddress
• ipv6 support with brackets

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• Rename peer to endpoint:
• option --peer/-p −> --endpoint/-ep
• import sys and silkaj.constants directly
• write tests

Kept the ability of not having click installed

• #354 The tests are difficult to write Actually, I am not sure if this feature is easy use. Using Silkaj as lib is kind of difficult since click is imported in almost every modules

• Introduce click context helper

Close #181 (closed), #409 (closed).