Allow the use of single newlines in help output

[bmesuere]

Due to the text reflow in the help output, single newlines are ignored, double newlines are used to indicate the start of a new paragraph and paragraphs are joined with a double newline. This means it's not possible to use a line break in the description without introducing an empty line. When using lists, this results in ugly output:

This command has 3 features:

- awesome feature 1

- awesome feature 2

- awesome feature 3

We also have a lot of options.

preferred output:

This command has 3 features:
- awesome feature 1
- awesome feature 2
- awesome feature 3

We also have a lot of options.

[denisdefreyne]

This is problematic to fix in a backwards-compatible way.

One idea I had was to parse the input as Markdown (kramdown to be precise) and convert the internal kramdown to something that is fit for terminal output.

[bmesuere]

While markdown support would be great, maybe it's an option to add a metacharacter that gets translated into a newline?

[denisdefreyne]

I just discovered tty-markdown, which might be a very good for this use case.

[esotericpig]

I think just a simple {n} would be nice that StringFormatter would look for.

To make it backwards compatible, just add an option in Command that is set to false by default.

This command has 3 features:{n}
- awesome feature 1{n}
- awesome feature 2{n}
- awesome feature 3{n}

We also have a lot of options.

super_cmd = @app_cmd = Cri::Command.define(newline_metachar: true) do
  # ...
end

super_cmd.define_command(newline_metachar: true) do
  # ...
end

This is easier (just parse for {n}; don't have to include the tty-markdown gem) and backwards-compatible (newline_metachar or w/e name is set to false by default).