Usage questions - remote & branch default values

Not an issue, but some clarifying questions to a new user:

1. Why does `remote_name` default to `deploy` as opposed to `origin`?
2. why does `branch` default to `master` as opposed to whatever branch is currently checked out (ie `${DRONE_BRANCH}`)?
3. `ssh_key` is mandatory right, ie. there's no way to tap into the authentication granted to drone instance itself?
4. why doesn't `remote` default to `${DRONE_GIT_SSH_URL}`?

This is so the pipeline step definition could be less verbose, assuming the most common use-case would be (commiting and) pushing back to same reporitory & branch.

Following drone step definition works, but feels like it could/should be more terse:
```
- name: git-push
image: appleboy/drone-git-push
settings:
ssh_key:
from_secret: github_ssh_key
branch: "${DRONE_BRANCH}"
remote: "${DRONE_GIT_SSH_URL}"
followtags: true
when:
branch:
- master
```

Review the plugin configuration represented by remote_name, branch, ssh_key, and remote, using the Drone step definition in the issue as the starting specification. Done means a maintainer decision on supported defaults and authentication behavior, with the resulting implementation or documentation scope identified; no files or tests are named.

Written by the indexing model from the issue text.