[New] nvm use/nvm install: add --save option

Fixes #2849.

Co-authored-by: Martin <maartin00000@gmail.com>
Co-authored-by: Jordan Harband <ljharb@gmail.com>

test/fast/Unit tests/Running 'nvm use --save' works with a .nvmrc in the parent dir

@@ -0,0 +1,57 @@
+#!/bin/sh
+\. ../../../nvm.sh
+\. ../../common.sh
+
+set -e
+
+TEST_VERSION="v0.2.4"
+
+if [ -f .nvmrc ]; then mv .nvmrc .nvmrc.orig; fi
+if [ -f ../.nvmrc ]; then mv ../.nvmrc ../.nvmrc.orig; fi
+
+del_nvmrc () {
+  rm -f .nvmrc ../.nvmrc
+}
+
+cleanup () {
+  del_nvmrc
+  nvm cache clear
+  nvm deactivate
+  nvm unalias default
+  rm -rf ${NVM_DIR}/v*
+  if [ -f .nvmrc.orig ]; then mv .nvmrc.orig .nvmrc; fi
+  if [ -f ../.nvmrc.orig ]; then mv ../.nvmrc.orig ../.nvmrc; fi
+  unset -f nvm_ls_remote nvm_ls_remote_iojs
+}
+
+die () {
+  echo "$@"
+  cleanup
+  exit 1
+}
+
+REMOTE="$PWD/mocks/nvm_ls_remote.txt"
+nvm_ls_remote() {
+  cat "$REMOTE"
+}
+REMOTE_IOJS="$PWD/mocks/nvm_ls_remote_iojs.txt"
+nvm_ls_remote_iojs() {
+  cat "$REMOTE_IOJS"
+}
+
+del_nvmrc
+make_fake_node "$TEST_VERSION"
+
+(cd ..
+nvm use --save "$TEST_VERSION" || die "\`nvm use --save $TEST_VERSION\` failed in the parent dir")
+nvm use --save || die "\`nvm use --save\` failed"
+
+[ -f ../.nvmrc ] && [ -f .nvmrc ] || die "expected two .nvmrc files to be generated"
+
+OUTPUT=$(cat .nvmrc)
+EXPECTED_OUTPUT="$(cat ../.nvmrc)"
+
+[ "_$OUTPUT" = "_$EXPECTED_OUTPUT" ] \
+  || die "invalid \`nvm use --save \` output: expected '$EXPECTED_OUTPUT'; got '$OUTPUT'"
+
+cleanup