Restore --stdin flag functionality

[lpedrosa]

Summary

This PR fixes the --stdin functionality which seemed to not work when the new version started using Argu for parsing the CLI arguments.

I have also preserved the original functionality where stdin input can only be read if redirected through a pipe:

# this would work
$ echo 'open System;; let () = printf "Hello World"' | fantomas-tool.exe --stdin
open System

let () = printf "Hello World"

# this would not
fantomas-tool.exe --stdin
Input path is missing...

Examples

Current master build:

$ echo 'open System;; let () = printf "Hello World"' | fantomas-tool.exe --stdin
ERROR: argument '--stdin' must be followed by <string>.
... omitted usage output

# when input is not redirected
$ fantomas-tool.exe --stdin
ERROR: argument '--stdin' must be followed by <string>.
... omitted usage output

Fantomas.CoreGlobalTool 3.1.0:

$ echo 'open System;; let () = printf "Hello World"' | dotnet fantomas --stdin
open System

let () = printf "Hello World"

# when input is not redirected
$ dotnet fantomas --stdin
Input path is missing...

This PR:

$ echo 'open System;; let () = printf "Hello World"' | fantomas-tool.exe --stdin
open System

let () = printf "Hello World"

# when input is not redirected
$ fantomas-tool.exe --stdin
Input path is missing...

Notes

The stdIn reader functionality is currently line capped (to 2000 lines, see StdLineLimit), to avoid memory overflows. The default Console.ReadLine() character limit is 256 characters, so that takes care of memory overflows.

We could potentially relax that limit, depending on how people use the --stdin functionality i.e. I am assuming this can be used as an editor hook, to format code selections
.

[nojaf]

Interesting, by using TryGetResult the current workaround for version can be simplified I think.
Something for another PR but thanks for this insight.

[nojaf]

We could potentially relax that limit, depending on how people use the --stdin functionality i.e. I am assuming this can be used as an editor hook, to format code selections

For now, there aren't that many people using stdin so I would not take this into account just yet.
Editors will not use the cli to format code but the Fantomas dll are integrate it that way.

Thanks for this PR @lpedrosa !