Trim whitespace in readme output

[SUPERCILEX]

I'm not sure if it's ok to trim whitespace in all output (since it'd be nice to have a tailing newline in some stdout file), so I'm thinking only trailing lines in readmes should be trimmed.

[epage]

The way I'm using this in clap, I very much want that trailing newline in the readme files. We had very inconsistent trailing newlines until we adopted trycmd.

[SUPERCILEX]

In readme files? Would you mind sharing an example of where having a blank line before the closing ``` is helpful?

[epage]

First, in github rendering, it hasn't caused harm in my READMEs, like cargo-edit

Second, I was referring to how it helped us catch clap bugs by having it in all of our examples, like this output.

[SUPERCILEX]

Ohhh, I see. You're saying that it should be consistent so you can catch whitespace bugs regardless of the position of the test. Makes sense to me.