Overwrite cli flag
Currently the cli will happy overwrite an existing transpiled file.

e.g.
`py2many.py --rust=1 /tmp/foo.py` will overwrite `/tmp/foo.rs`

The same occurs if `--outdir` is used.

And in directory mode, this can cause a "target language project build phase" to go haywire, as there may be some stale generated files in the directory, left over from a previous transpilation.

https://github.com/adsharma/py2many/pull/350#pullrequestreview-694194375 makes this even more troublesome, as it would overwrite the input file.

Often forced overwrite is a `--force` flag.  Sometimes a shortopt of `-f` is allowed.