backup flag does not produce a toml file

[okrus]

Issue Name

backup flag does not produce a toml file

Overview

1. OS name and version.

mac OS catalina 10.15.6 (19G2021)

2. go version output

go version go1.13.7 darwin/amd64

3. waflyctl --version in recent releases or brew info waflyctl output.

dev

4. Description of the problem. What is happening that shouldn't?

when using waflyctl --backup flag the output file is not in toml format so if you need to use this file to restore your waf to a previous state, before you need to parse the file so it has the right format

5. Description of the expected behaviour.

it would be super helpful to produce a backup file that could be consumed without any parsing or tweaking

6. Exact steps to reproduce the error.

you can run the following command to generate a backup file of your waf
waflyctl --apikey $token --serviceid $s_id --backup --backup-path=/path/waf_config.bak

Additional Information:

See your reference a sample of a TOML config file here:
https://github.com/fastly/waflyctl/blob/master/config_examples/waflyctl.toml.example