Add helper for SSH setup

[mathiasbynens]

Every repo using travis-scripts using deploy keys (as recommended) will include some boilerplate code like this:

# Decrypt the file containing the private key.

openssl aes-256-cbc \
    -K $encrypted_fa862907f31d_key \
    -iv $encrypted_fa862907f31d_iv \
    -in "$(dirname "${BASH_SOURCE}")/${PRIVATE_KEY_FILE_NAME}.enc" \
    -out ~/.ssh/${PRIVATE_KEY_FILE_NAME} -d;

# - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

# Enable SSH authentication.

chmod 600 ~/.ssh/${PRIVATE_KEY_FILE_NAME};
echo 'Host github.com' >> ~/.ssh/config;
echo "  IdentityFile ~/.ssh/${PRIVATE_KEY_FILE_NAME}" >> ~/.ssh/config;

Could travis-scripts include a helper utility that performs these tasks? The only variables are the encrypted key file name, and the key and IV variables. That would allow replacing the above with something like:

set-up-ssh --encrypted-key-file "github_deploy_key" \
           --key "$encrypted_fa862907f31d_key" \
           --iv "$encrypted_fa862907f31d_iv"

This would greatly simplify this section in the docs (ref. #20): https://github.com/alrra/travis-scripts/blob/master/doc/github-deploy-keys.md#26-setup-the-ssh-key-for-travis-ci