Identifying input files

The goal is to be able to re-run the experiment but while replacing the input files with others. To do that:
* [X] reprozip needs to identify what these input files are
* [x] they need to be written to the configuration (where the user can add/remove/label them)
* [ ] we need some kind of UI to specify we want to replace these with others. This will probably be done once we run the experiment from reprounzip (instead of shell script; this is #46). It is also a requirement for #8.