Add option to skip binary files

`./bin/k2pdfopt_linux` is a binary file which should not be touched:

```
repren --dry-run --literal --from agC --to ag_c .
Dry run: No files will be changed
Using 1 patterns:
'agC' -> 'ag_c'
Found 200 files in: .
- modify: ./bin/k2pdfopt_linux: 1 matches
- modify: ./zsh/auto-load/others/ag.zsh: 3 matches
Read 200 files (14410423 chars), found 4 matches (0 skipped due to overlaps)
Dry run: Would have changed 2 files (2 rewritten and 0 renamed)
```