nimpretty shall support pipe on stdin, to stdout
### Summary

nimpretty currently must run on nim files (passed as arguments),

and lacks the feature of `pipe`.

### Description

Formatter of other langs, like rustfmt, gofmt, allows reading stdin and outputting to stdout,

which is handy, for example, in vim, you can simply use `:%!rustfmt` to format current source code file.

Yet neither `:%!nimpretty` or `:%!nimpretty -` doesn't work, as it currently expects one or more files as arguments.

### Alternatives

Write to a temp file, nimpretty it and then cat it.

In addition, according to https://github.com/nim-lang/Nim/issues/8458#issuecomment-655082973, relying on bash substitution won't work


### Examples

_No response_

### Backwards Compatibility

None.

This just allows a cli usage used to be invalid

### Links

https://github.com/nim-lang/Nim/issues/8458#issuecomment-508845461