Add support for config file as an alternative to Environment variables

[Stretch96]

When running the empire server the configuration is loaded through either flags or environment variables.

The latest codegansta/cli (now urfave/cli) package has added support for TOML/YAML files.

I've created a PR to update the cli package: #1116

Having the ability to load the configuration from a file instead of environment variables would make configuration management a little easier I believe.

[Stretch96]

@ejholmes Looks as though it'll need to change again to gopkg.in/urfave/cli.v1 instead of github.com/urfave/cli 🤦‍♂️

Whilst trying to add the wrapper to allow the flags to be defined as an alternative source, the current package throws type errors: urfave/cli#561

I'll have a play around, I may just be doing it wrong. I was trying:

[...]
        altsrc.NewStringFlag(
                cli.StringFlag{
                        Name:   FlagURL,
                        Value:  "",
                        Usage:  "That base URL where this Empire instance runs",
                        EnvVar: "EMPIRE_URL",
                },
        ),
[...]

With github.com/urfave/cli, it throws type errors. With gopkg.in/urfave/cli.v1, the flags don't get initialised 🤷‍♂️