Support multiple inputs

[ESultanik]

We currently have support on the database side for having multiple inputs. However, we need to:

• [x] ~Add an additional database table to track multiple runs of the program. This will allow us to differentiate between runs when we eventually add support for tracking multiple inputs in a single run.~ We decided that it's probably better to have one DB file per run
• [x] Allow for multiple inputs
• [x] Allow POLYPATH to include a colon-delimited list of paths to files to track
• [x] No longer require POLYPATH. If omitted, then track all inputs. If set either in an environment variable or in a config file, only track the files listed in POLYPATH
• [ ] Add support for this in the Python API.
• [ ] optional: Exit with non-zero exit code and an error message if the database file already exists
• [ ] optional: Add some sort of "force overwrite" flag that will allow the db to be overwritten if it already exists
• [ ] Remove this exception once we add support: https://github.com/trailofbits/polytracker/blob/b9f095ac39a5c198e49c32f315c14c9b5197522e/polytracker/database.py#L451-L456