Update zooom function to support refactored chartSeries function

[erichung0404]

Since S4 method used by chartSeries is deprecated, xdata and xsubset should now be obtained from .plotxtsEnv environment. plot object needs to be returned by calling return() or being executed at the end of the code otherwise it will not be printed. To print both the plot object and "done" message, print() is used. But passing the plot object created by zooom() to an object is not supported.

data("sample_matrix")
x <- sample_matrix
chartSeries(x)
x.subset <- zooom()
x.subset
# NULL