Restore in flag (#33)

* fix: restore functionality of -in flag

Originally I had changed the read from stdin to be specified by a single
- argument or /dev/stdin by suggestion from #26. In that PR I had
removed the -in flag. After sleeping on it I realized I might as well
keep that flag, since it might intuitively make sense to more people,
makes more sense on Windows, and maintains backwards compatibility (even
though that is currently not a promise of the project, might as well do
it if it's easy).

* docs: add -in flag back to README

README.md

@@ -65,7 +65,7 @@ The CLI supports the following flags/arguments:
 	- Format the matched files and output the diff to `stdout`
 * Lint (`-lint` flag)
 	- Format the matched files and output the diff to `stdout`, exits with status 1 if there are any differences
-* Stdin (just `-` or `/dev/stdin` argument)
+* Stdin (just `-` or `/dev/stdin` argument, or `-in` flag)
 	- Format the yaml data from `stdin` and output the result to `stdout`
 * Custom config path (`-conf` flag)
 	- If you would like to use a config not stored at `.yamlfmt` in the working directory, you can pass a relative or absolute path to a separate configuration file

cmd/yamlfmt/flags.go

@@ -28,11 +28,12 @@ var (
 source yaml and formatted yaml.`)
 	flagDry *bool = flag.Bool("dry", false, `Perform a dry run; show the output of a formatting
 operation without performing it.`)
+	flagIn   *bool   = flag.Bool("in", false, "Format yaml read from stdin and output to stdout")
 	flagConf *string = flag.String("conf", "", "Read yamlfmt config from this path")
 )
 
 func getOperation() command.Operation {
-	if len(flag.Args()) == 1 && isStdin(flag.Args()[0]) {
+	if *flagIn || isStdinArg() {
 		return command.OperationStdin
 	}
 	if *flagLint {
@@ -44,7 +45,11 @@ func getOperation() command.Operation {
 	return command.OperationFormat
 }
 
-func isStdin(arg string) bool {
+func isStdinArg() bool {
+	if len(flag.Args()) != 1 {
+		return false
+	}
+	arg := flag.Args()[0]
 	return arg == "-" || arg == "/dev/stdin"
 }