comBGC: add optional input parameter

[jasmezz]

Description of feature

As discussed in this thread in the funcscan Slack channel, it would be nice to be able to input a whole directory with BGC tool output files (with subdirectories of samples) instead of single files (as is the only option at the moment). Example output structure for antiSMASH that should be accepted as input:

antismash/
├── MAG001/
|   ├── index.html
|   ├── knownclusterblast/
|   └── more-antismash-output
├── MAG002/
|   ├── index.html
|   ├── knownclusterblast/
|   ├── more-antismash-output
└── MAG.../
    ├── index.html
    ├── knownclusterblast/
    └── more-antismash-output