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