Wait for debugger flag for .NET Core 2.0 / 2.1 implementation

[ndobryanskyy]

Original issue

125 - issue, that requests .NET Core 2.0 / 2.1 debugging support

Changes

• Added DebuggerExtensions class which enables waiting for the debugger to attach in the loop, inspecting Debugger.IsAttached property with interval (50ms). Also it has 10 minutes timeout, after it, function will continue executing without debugging;
• Added --debugger-spin-wait flag handling, if it supplied as an argument, then, we will use DebuggerExtensions.TryWaitForAttaching to wait for the debugger before proceeding to execution;
• Added try / catch block for all body of Main to prevent user from hanging on External Code Exception while debugging.

Side note

I've closed #127 as it had a lot of unnecessary stuff. Now this PR has only required changes, to proceed with #125.

[mhart]

Thanks so much for this @ndobryanskyy – it looks great!

I've already pushed images up at lambci/lambda:dotnetcore2.0 and lambci/lambda:dotnetcore2.1 with these changes – have fun debugging!