cmd/principal: Add --file flag to recvblessings and bless.

As per https://github.com/veyron/release-issues/issues/946,
users can now run

principal recvblessings --file foo

The remote_key, remote_token, and principal will be writte to that file,
Then users can invoke

principal bless --file foo extension

to successfully get a blessing.

Change-Id: Icc47c8550f4e5c61e8ff646bf176075e6f453768
3 files changed