Close#21.
Refactor environment variables to configuration file (config.yaml) and registration file (.runner).
The old environment variables are still supported, but warning logs will be printed.
Like:
```text
$ GITEA_DEBUG=true ./act_runner -c config.yaml daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG has been ignored because config file is used
$ GITEA_DEBUG=true ./act_runner daemon
INFO[0000] Starting runner daemon
WARN[0000] env GITEA_DEBUG will be deprecated, please use config file instead
```
Reviewed-on: https://gitea.com/gitea/act_runner/pulls/90
Reviewed-by: Lunny Xiao <xiaolunwen@gmail.com>
Jason Song