Add null-ckeck analyzer and code fix

[s-arash]

-Add the analyzer and code fix classes -Add related unit tests -Modify readme.md to showcase the new feature

The analyzer looks for cases like this:

if ( handle != null) handle.Dispose();

and the code fix converts them to:

handle?.Dispose();

It recognizes more complex syntax too.

if ( repo.Customers != null) repo.Customers[0].Orders.Verify();

--->

repo.Customers?[0].Orders.Verify();

[DustinCampbell]

@s-arash, my apologies for not getting to this sooner! FWIW, we actually delivered this feature in the last update to Visual Studio 2015. So, you shouldn't need it C# Essentials.

[s-arash]

@DustinCampbell as far as I know, Visual Studio only looks for delegate invocation

if (SomethingHappened != null) SomethingHappened(this, args); 
// -->
SomethingHappened?.Invoke(this,args);

This code fix looks for other cases where null conditional access can be used to simplify code. Unless I'm missing something obvious, that is the situation.

[DustinCampbell]

Ah yes, you are correct. Sorry about that!

[s-arash]

No problem!

[normanhh3]

Is this going to get merged in?!