Add support for setting options and exporting environment variables

[rajatjain1997]

Something close to the following syntax can be supported to set bash-like options and export environment variables:

{
    subprocess::set(pipefail);
    subprocess::export("var_name", "var_value");
    return (subprocess::command{"random_cmd"} | subprocess::command{"another_cmd"}).run();
}
// pipefail is automatically unset
// var_name is automatically unset