Option to globally disable colour output

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/1345)
In GitLab by [[Gitlab user @rjek]](https://gitlab.com/rjek) on Jun 29, 2020, 10:41

## Summary

[//]: # (Summarize the bug encountered concisely)

There is apparently `--no-color` but there is no way to globally disable colour output from, say, a dotfile in my home directory. Colour output is actively unhelpful for the 10% of the population that have some degree of colourblindness.

## Steps to reproduce

[//]: # (How one can reproduce the issue - this is very important)

Run any BuildStream command.

## What is the current bug behavior?

[//]: # (What actually happens)

There is colour.

## What is the expected correct behavior?

[//]: # (What you should see instead)

Method to globally disable colour without having to pass --no-color to every invocation.

## Relevant logs and/or screenshots

[//]: # (Paste any relevant logs - please use code blocks ``` to format console output, logs, and code as it's hard to read otherwise. You can also add a Snippet and link it here. Check the markdown giude on Gitlab for further tips)

## Possible fixes

[//]: # (If you can, link to the line of code that might be responsible for the problem)

## Other relevant information

[//]: # (Add the BuildStream version. Substitute below the "x" by "1", "2" or the right version. Add more than one version if necessary)

* BuildStream version affected: /milestone %BuildStream_v1.x

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Start by locating the existing --no-color command-line option and the handling of BuildStream command configuration. Add a home-directory setting that disables colour for all invocations, while preserving the explicit option, and verify that commands produce no colour when the global setting is enabled.

Written by the indexing model from the issue text.