Allow getAvailableBrowsers to pass options to which module

Hi, thanks for this library!

I'm playing with it inside an Electron app and I've encountered a minor issue (with Electron). Short story: the Electron renderer process.env doesn't inherit the variables from the main process on Linux, which are available through renderer.process.env. So I have to use the which options to pass the PATH env variable.

I would basically like to do something like:

let opt = { path: remote.process.env.PATH };
detectBrowsers.getAvailableBrowsers(opt);

Which can be achieved just adding the optional parameter to getAvailableBrowsers:

async function getExecutable(browser, opt) {
  const execs = browsers[browser];
  if(!execs || !execs[process.platform]) return;
  for(const exec of execs[process.platform]) {
    const executable = await which(exec, opt).catch(()=>{});
    if(executable) return executable;
  }
}

async function getAvailableBrowsers(opt) {
  const list = [];
  for(const browser of Object.keys(browsers)) {
    const exec = await getExecutable(browser, opt);
    if(exec) list.push({ browser, path: exec });
  }
  return list;
}

I guess passing options to the which module could be useful in some other cases, so it would be nice if a future release included the ability to do it somehow.