Optionally read GPG key passphrase from file

gpg supports reading a key's passphrase from a specified file. This
commit adds a new configuration option `GPG_PASSPHRASE_FILE` and related
commandline options `-p|--passphrase-file` to `freight-cache` which
allow specifying the file from which to read the GPG key's passphrase
when signing the Release file. Allows for running Freight
non-interactively and without asking for user input which can be
desirable in certain situations. Addresses issue #33.