Add BeginCapture with a name of group and method to retrieve value captured.

· Add BeginCapture with a name of group.
· Add a Capture method to retrieve value of a name group.

Example:
To extract 123 from this string "COD123END"

verbEx.Add("COD")
                .BeginCapture("GroupNumber")
                .Any("0-9")
                .RepeatPrevious(3)
                .EndCapture()
                .Add("END");

verbEx.Capture("COD123END","GroupNumber") // return 123