Allow setting window size in spawned process.

[kosayoda]

My use-case for this is for testing a shell that uses the rustyline crate for line-editing.
For long inputs (greater than the terminal width), rustyline inserts newlines into its output, which breaks tests using rexpect to match expected strings.
This PR would allow users to for example, set the terminal size to 200 columns wide, avoiding the aforementioned problem.
The feature is available in pexpect.

Coincidentally, this PR addresses the newly opened #119.

Draft Status

The PR is in draft because I am not sure what the best way to go about passing the Options is.
Currently it already feels weird to me propagating the reader::Options from spawn_with_options down the various containers to the NBReader.