Allow the user to supply a lambda/Proc as the :matching paramater

Allow the user to supply a lambda/Proc as the :matching paramater.  If
this function returns true then catch the error, false raise the error.

I thought this was a useful addition to retryable because I ran into a problem where if I wanted to supply a regex for one exception type it was applied to all exception types.  This change allows the user to supply a lambda/proc that can contain all the logic to decide if they want to retry a specific exception.