HTML format reports "Bad Request: can't parse entities: Unsupported start tag"

```
pipeline:
telegram:
image: appleboy/drone-telegram
pull: true
format: html
secrets: [telegram_to, telegram_token]
```

results in:
```
2018/09/26 15:42:35 Bad Request: can't parse entities: Unsupported start tag "https://drone.git.example.com/example/example/25" at byte offset 10
```

Start by reproducing the reported pipeline configuration with format set to html and inspect the code path that formats and sends the Telegram notification. Trace how the example URL becomes part of the HTML request and verify the result against Telegram's entity parsing error. Done means notifications containing that URL are accepted without the unsupported start tag error.

Written by the indexing model from the issue text.