feat(cli): add the --stdin-filepath option

This option is really usefull when using prettier-eslint-cli inside text
editor plugins, to let prettier-eslint discover the correct eslintrc
configuration for the given file.

Related:
* prettier#95
* sbdchd/neoformat#112
* prettier/prettier-emacs#10 (comment)

src/format-files.js

@@ -30,6 +30,7 @@ function formatFilesFromArgv({
   _: fileGlobs,
   logLevel = logger.getLevel(),
   stdin,
+  stdinFilepath,
   write,
   eslintPath,
   prettierPath,
@@ -48,7 +49,7 @@ function formatFilesFromArgv({
   }
   const cliOptions = {write}
   if (stdin) {
-    return formatStdin(prettierESLintOptions)
+    return formatStdin({filePath: stdinFilepath, ...prettierESLintOptions})
   } else {
     return formatFilesFromGlobs(
       fileGlobs,

src/parser.js

@@ -22,6 +22,10 @@ const parser = yargs
       type: 'boolean',
     },
     stdin: {default: false, describe: 'Read input via stdin', type: 'boolean'},
+    'stdin-filepath': {
+      describe: 'Path to the file to pretend that stdin comes from.',
+      coerce: coercePath,
+    },
     'eslint-ignore': {
       default: true,
       type: 'boolean',