Add --out-file-extension option to babel-cli (#9144)

* Add --use-file-extension option to babel-cli

* refactor file extension logic

use existing methods from path module

* Rename use-file-extension -> out-file-extension

* add error message if bot keep- and out-file-extension is used

* add test for error when mixing --keep- and --out-file-extension

* poke travis

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/in-files/src/bar.mjs

@@ -0,0 +1 @@
+arr.map(x => x / DIVIDER);

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/in-files/src/foo.jsx

@@ -0,0 +1 @@
+arr.map(x => x * MULTIPLIER);

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/options.json

@@ -0,0 +1,12 @@
+{
+  "args": [
+    "src",
+    "--out-dir",
+    "lib",
+    "--out-file-extension",
+    ".mjs",
+    "--extensions",
+    ".jsx,.mjs",
+    "--verbose"
+  ]
+}

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/out-files/lib/bar.mjs

@@ -0,0 +1,5 @@
+"use strict";
+
+arr.map(function (x) {
+  return x / DIVIDER;
+});

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/out-files/lib/foo.mjs

@@ -0,0 +1,5 @@
+"use strict";
+
+arr.map(function (x) {
+  return x * MULTIPLIER;
+});

packages/babel-cli/test/fixtures/babel/dir --out-dir --out-file-extension/stdout.txt

@@ -0,0 +1,3 @@
+src/bar.mjs -> lib/bar.mjs
+src/foo.jsx -> lib/foo.mjs
+Successfully compiled 2 files with Babel.

packages/babel-cli/test/fixtures/babel/error --keep-file-extension --out-file-extension/options.json

@@ -0,0 +1,4 @@
+{
+  "args": ["--keep-file-extension", "--out-file-extension", ".mjs"],
+  "stderrContains": true
+}

packages/babel-cli/test/fixtures/babel/error --keep-file-extension --out-file-extension/stderr.txt

@@ -0,0 +1 @@
+--out-file-extension cannot be used with --keep-file-extension