Prevent --terminal-width=0 and invalid forms of --terminal-width.

[eth-p]

My proposed solution to #535, which involves validating --terminal-width to report an error if somebody provides a width of zero.

Why this approach, and not having it disable wrapping?

User convenience is definitely something to consider, but in this instance I believe having --terminal-width=0 being analogous to --wrap=never would be more of a quirky behaviour than a quality-of-life feature.

1. Having such a feature would dilute the meaning of --terminal-width.
2. Using --terminal-width=0 as --wrap=never isn't very clear or intuitive.
3. Using --terminal-width=0 as a way to disable wrapping results in a conflict when you don't want wrapping, but do want to specify a specific terminal width.
4. It's more explicit (and less characters) to type --wrap=never.

[sharkdp]

Thank you for your contribution and the detailed description. Fully agree with your analysis. I've added two minor inline comments.

[sharkdp]

Thank you for the update!