Experiment with building a sensible PathMapper.file_mapper default directly into the decorator

I think the simplest possible plugin shouldn't need to include this detail, instead having sensible defaults.

i.e.
```py
@chris_plugin(
parser=parser,
)
def main(options: Namespace, inputdir: Path, outputdir: Path):
"""
mapper = PathMapper.file_mapper(inputdir, outputdir, glob="*")
for input_file, output_file in mapper:
pass
```

Should be achievable with just this:
```py
@chris_plugin(
parser=parser,
)
def main(options: Namespace, inputdir: Path, outputdir: Path):
"""
for input_file in os.listdir(inputdir)
pass
```

i.e. treat the paths given as if it was a simple linux script. Or:
- have `main` take the input_file and output_file one at a time, and repeatly call the function
- have `main` take mapper directly as an arg.

I am unsure which I prefer. I would lean towards main taking mapper directly as an arg, which would better self-describe how it is meant to work. I think inputdir and outputdir at present are misleading slightly.