Refactor environment variables to configuration and registration (#90)

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 <[email protected]>

.gitignore

@@ -3,6 +3,7 @@ act_runner
 .runner
 coverage.txt
 /gitea-vet
+/config.yaml
 
 # MS VSCode
 .vscode