Implement accessibility mode

[AntonHermann]

Fixes #194

This is my take at implementing an accessible command line interface.

When either setting the environment variable ACCESSIBLE (to an arbitrary value) or passing the CLI flag --accessible/-A, execute in accessibility mode: reduce visual noise and unnecessary output (almost all info!() calls)

When setting the environment variable ACCESSIBLE to a nonempty value when executing install.sh, suppress noisy output of curl/wget

[AntonHermann]

Sorry about the long delay, I've been pretty busy recently.

This PR contains a fixup commit now which doesn't add any value of its own and there is a small conflict with master. How do you want to handle this?

I could create a merge commit, but I think squashing and rebasing all commits onto master would be cleaner. I would await another review and then squash, rebase and force-push this PR? This way we only have a single, significant commit being merged into master.

[marcospb19]

I could create a merge commit, but I think squashing and rebasing all commits onto master would be cleaner.

Whatever you'd prefer! So far, I did not spent much time on making git history clear, I know it's good, but sometimes I think that we are just low on time, and should pick the "less effort" alternative.

About the conflicts here, I'll solve them once the PR is ready, do not worry.

EDIT: no worries for the delay too, we really appreciate the contribution.

[AntonHermann]

Okay great.
I'd like to clear up my doubts about #197 (comment), then I will clean up the PR :)