Export Subcommand

[ReeseHatfield]

Add subcommand for PDF exporting.

oseda export [optional port] [optional filename]

#[derive(Args, Debug, Clone)]
pub struct ExportOptions {
    /// String name of the output PDF file
    #[arg(long, default_value = "slides.pdf")]
    pub output: String,
    /// Port the project runs on
    #[arg(long, default_value_t = 3000)]
    pub port: u16,
}

Also rewrote the run command to support killing via an Arc