modification of the apps functions

Hi, I am using h5pyd in a higher-level program. The logic implemented in the so called apps is interesting. I think they would be useful as functions that take parameters to be reused in other programms.
What do you think ?
Here a listed the two main modifications I think must be done:

- [ ] stateless: init cfg structure in the main function and put it as a parameter to the functions (PR #91)
- [ ] arguments: use the argparse package to handle arguments
- [ ] transform the logic implented in the main functions of the apps in functions that take parameters (the same as those from the argv) from a new main function.

I can contribute via a PR if you are interested.