Fix custom plot label option with filenames

[trevortknguyen]

This shouldn't change any other part of the program. It only intends to fix the fact that the --plot-label argument in sashimi_plot was not working properly.

I believe the ext variable meant to store the file extension, so it should be changed to the second index of the result from splitting the file on a period.

I know that on line 94, the dirname method is most likely not written correctly because it only accepts one parameter. https://docs.python.org/2.7/library/os.path.html#os.path.dirname

With my changes, I was able to get the plot-label to work allowing me to specify a new filename for each plot.