Malformed escapes cause panic

[gandalf3]

Malformed escape codes (e.g. just ESC[) cause a panic. Here is a minimal example:

use cansi::v3::categorise_text;

fn main() {
    let x = categorise_text("oops\x1b[\n");
    println!("{}", x[0].text);
}

I believe at least from Oxker's perspective it'd make the most sense for cansi to ignore these and remove them from the resultant slice (ignoring them seems to be what most terminal emulators do).

I understand if you are not interested in fixing this, but for context I'm here from this issue: mrjackwills/oxker#17