Add new features in BashScriptor

[prioux]

BashScriptor is a built-in tool in the CBRAIN platform; for obvious reasons it should only be run by admins.

I find that sometimes the basic capabilities are not enough for mass-processing of some datasets. I'd like:

• Having a global counter available on the subtasks, when serializing the processing for files in bunches
• Having a counter for the subtasks themselves
• Having a local prologue and epilogue script
• The option to make all the tasks depend on each other (e.g. task 2 starts only when task 1 is finished)

[prioux]

Maybe also special substitution tokens for "first task", "last task", "first file" and "last files" instead of comparing the counters to 1 and N.