Mac M1 support

[nathanbabcock]

Fixes #15

[jt-wang]

@nathanbabcock Thanks Nathan!

 ~/github/ffmpeg-sidecar main v0.4.1 git checkout feat/mac-m1   
branch 'feat/mac-m1' set up to track 'origin/feat/mac-m1'.
Switched to a new branch 'feat/mac-m1'
 ~/github/ffmpeg-sidecar    cargo run --example download_ffmpeg
   Compiling ffmpeg-sidecar v0.5.0 (~/projects/ffmpeg-sidecar)
    Finished dev [unoptimized + debuginfo] target(s) in 1.24s
     Running `target/debug/examples/download_ffmpeg`
FFmpeg is already installed! 🎉
For demo purposes, we'll re-download and unpack it anyway.
TIP: Use `auto_download()` to skip manual customization.
Error: Error { message: "Downloads must be manually provided for non-x86_64 architectures", source: None }

I believe the Error is from line 18,

pub fn ffmpeg_manifest_url() -> Result<&'static str> {
  if cfg!(not(target_arch = "x86_64")) {
    return Err(Error::msg(
      "Downloads must be manually provided for non-x86_64 architectures",
    ));
  }

because ffmpeg_manifest_url is called in line 147,

pub fn check_latest_version() -> Result<String> {
  let string = curl(ffmpeg_manifest_url()?)?;

  if cfg!(target_os = "windows") {
    Ok(string)
  } else if cfg!(target_os = "macos") {
    Ok(parse_macos_version(&string).ok_or("failed to parse version number (macos variant)")?)
  } else if cfg!(target_os = "linux") {
    Ok(parse_linux_version(&string).ok_or("failed to parse version number (linux variant)")?)
  } else {
    Err(Error::msg("Unsupported platform"))
  }
}

[nathanbabcock]

@jt-wang you are completely correct of course, and my apologies for taking a while to update the PR.

Give it another go now with the same command.