Add --context flag

[cykl]

Description

This patch adds an optional --context global flag.
Setting --context overrides the current context to use for this command.
Its behavior is similar to kubectl --context.

Fixes #214

Type of change

• New feature (non-breaking change which adds functionality)

Documentation

• the change is mentioned in the ## [Unreleased] section of CHANGELOG.md
• the configuration yaml was changed and the example config in README.adoc was updated
• a usage example was added to README.adoc
• tests for the changes have been implemented (see: Testing your changes)

[cykl]

@d-rk I'm learning go on the fly, consider everything I do to be foolish.

I'm not happy that I didn't figure out how to unit test my changes.
Any help is welcome.

I did manually test my changes which seems OK.
Setting --context indeed overrides the default (current) context.
Auto-completion seems OK too

./kafkactl  __complete --context  ""
aws-dev
aws-preprod
aws-prod
aws-staging
:0
Completion ended with directive: ShellCompDirectiveDefault

I'm not sure if it worth mentioning in the README. Any opinion?

[drawks]

This is great, I was going to create an issue and PR for this exact thing, any chance we can get a code review/merge here? @d-rk

[d-rk]

@cykl sorry for the long wait. I add a small refactoring and some tests.
Testing is sadly not always that intuitive in this repo. Perhaps I'll find the time to improve on this sometime.
Anyway, thank you for your contribution 👍

[cykl]

Thanks @d-rk! No worries, we all have busy lives.