Suggest multiple line change within a merge request
It is useful to suggest a simple single line substitution, but there are many situations where a single line substitution is not sufficient. We should support multiple line suggestions too.
In https://gitlab.com/gitlab-org/gitlab-ce/issues/18008 the
```suggestion code block marker was introduced to allow single line suggestions.
We should extend this with support for specifying a range for replacement like
```suggestion:L2-4 which means replace lines 2-4 with this code block.