Overflow when scrollback > terminal rows

[chris-laplante]

vt100 is misbehaving if you set scrollback > terminal rows. Please see #4. If you run cargo test scrollback, you'll get this:

running 2 tests
test scrollback_larger_than_terminal ... FAILED
test scrollback ... ok

failures:

---- scrollback_larger_than_terminal stdout ----
thread 'scrollback_larger_than_terminal' panicked at 'attempt to subtract with overflow', src/grid.rs:125:42
note: run with `RUST_BACKTRACE=1` environment variable to display a backtrace