Implemented a function for developers to specify the `--depth` option of the `git clone` command

Implemented a function for developers to specify the --depth option of the git clone command. refs #55

I included the #60 instructions.

I made Git.Depth type `*int` so that we can distinguish the following cases:

* `git: depth:` is not set on .drone.yml. (In this case the default depth value(50) is set to --depth.)
* `git: depth:` is set to 0 on .drone.yml. (In this case 0 is set to --depth.)

```
type Git struct {
   Depth *int `yaml:"depth"`
}
```