Add support to deprecate aliases

[patriksvensson]

Right now it's not possible to deprecate aliases in a nice way.
It should be possible to mark an alias with an Obsolete attribute.

[CakeMethodAlias]
[Obsolete("You should use DoNewMagic instead.", false)]
public static void DoMagic(this ICakeContext context)
{
}

Would result in a warning message when running:

Warning: The alias DoMagic is obsolete. You should use DoNewMagic instead.

If the Obsolete attribute is configured to be an error, an exception should be thrown instead with the same message as the warning.

[patriksvensson]

@devlead @gep13 Would appreciate if one of you took a look at this 😄