Allow customising number of lines shown before <n lines omitted>

[gibfahn]

Often in CI I find the important details are lost behind:

```
<72 lines omitted>
```

this makes it hard to debug issues. It would be great to have an environment variable or similar to customise how much is printed.

[epage]

I've been hesitant to include env variables for controlling things, so far. Unsure if I'm more open to that or a feature flag.

[gibfahn]

A feature flag that disables truncating would be great for me if that works for you.

[epage]

I'm also not a big fan of feature flags, so that was a bit of "which evil do I prefer?"

[gibfahn]

Fair enough.

I keep hitting issues where truncated output hides the key details of what went wrong in CI, so I'm also weighing up using a git fork (not ideal) vs not using the crate, vs waiting for this to be an option.

I guess this could also be a set of options on the Command struct, so I could do something like:

let mut cmd = Command::cargo_bin("mycli")?;
cmd.truncate(false);
// ...

[gibfahn]

An alternative would be an option to print the full stdout and stderr to a temp file somewhere, so when the output is small one could just see it inline, but when it is large one could go find it if needed.