publish: github release plugin

I've PR'd this against upstream drone, but it's stagnating. I'd like to start testing this in our drone setup.

nit: This condition could be simplified as
```go
p.Github != nil && (len(p.Github.Branch) == 0 || r.Branch == p.Github.Branch)
```
Also, I might compare the actual value of the string instead of the length of it: 
```
p.Github != nil && (p.Github.Branch == "" || p.Github.Branch == r.Branch)
```