Support for OUTPUT variable arguments to procedure calls #268

Microsoft SQLServer has support for output variable arguments to procedure calls.
Suppose I have a procedure myProc which has two arguments: an input variable and an output variable. This procedure can be called like this:
EXECUTE myProc 'foo', @outputVar OUTPUT

More about this can be found here:

• https://technet.microsoft.com/nl-nl/library/ms187004.aspx
• http://stackoverflow.com/questions/1589466/execute-stored-procedure-with-an-output-parameter

The above query gives a ParseException in JSqlParser.

I am not quite sure how this should be implemented. I guess net.sf.jsqlparser.expression.UserVariable could have a boolean flag isOutputParameter but this wouldn't be exactly right, since it only applies to procedure call arguments and not to variables in general.
Maybe better would be to introduce an net.sf.jsqlparser.expression.ExecuteArgument object which implements net.sf.jsqlparser.expression.Expression, and which contains an expression and a isOutputParameter boolean flag.
In addition, net.sf.jsqlparser.statement.execute.Execute.getExprList() could then return a List<ExecuteArgument> rather than an ExpressionList.
What do you think about this, Tobias?

I am using JSqlParser version 0.9.5.