Update Decipherer

[toolboc]

*Various optimizations
*Decipherer follows strategy employed by
https://github.com/gantt/downloadyoutube
*Resolves #242

This Decipherer is based on https://github.com/gantt/downloadyoutube which was one of the very first MP4 download scripts that took on encrypted video signatures. It is also one of the fastest to update during breaking changes. If we shift to the strategy employed by @gantt , we can benefit from these properties if/when things break again.

[jphellemons]

Perhaps add an if statement to catch if the regex does not match. I had a non matching regex on line 23 for signatureFunctionName so I modified the FindMatch https://github.com/toolboc/YoutubeExtractor/blob/cc80466d708aa4d475b7a9917d726d0c62812d37/YoutubeExtractor/YoutubeExtractor/Decipherer.cs#L125

[jphellemons]

or remove the if and make it a one-liner with return matches.Count > 0 ? matches[0].Groups[1].Value : string.Empty;

private static string FindMatch(string text, string regexp)
{
    Regex rgx = new Regex(regexp);
    var matches = rgx.Matches(text);

    if (matches.Count > 0)
        return matches[0].Groups[1].Value;
    else
        return string.Empty;
}

[jphellemons]

#296 #301

[toolboc]

*feedback addressed in 75564dd

@jphellemons , please let me know if you see anything else =)