enhancements to VariablesFromCSV plugin

Enhanced the VariablesFromCSV plugin to make it easier to import variables for a test plan from a file which includes a header row and more than just two columns. This helps facilitate self-documenting files:

name,value,description
group0_threads,10,number of threads for thread group 0
group0_iterations,100,number of iterations per thread for thread group 0

The enhanced plugin will ignore every column after the second, and has an option to specify how many initial lines to skip in the input file.