fs-ci: open branch's CI page in browser

Requires additional configuration in project's `.fs.yml`:

```yaml
# .fs.yml
ci: "https://semaphoreapp.com/fs/rails-base/branches/%{branch}"
```

TODO:

* [x] docs
* [x] autocomplete branch name
* [x] error when CI URL is not set in `.fs.yml` (or it even doesn't exist)