Line break in description leads to wrong output

### MWE:

``` r
sys = modules::import('klmr/sys')

"
foo
bar
"

sys$run({
args = sys$cmd$parse(arg('input', '', ''))
})
```

Leads to the following output when executed via `Rscript scriptpath --help`:

```
\nDescription\nfoobar\n

Usage: scriptpath [input]

Positional arguments:
input (default: "")
```

This contains two errors:
- Newlines (and other special chars) should be correctly parsed in the description string.
- Leading and trailing whitespace (including newlines) should be trimmed.