Add automated check for broken include templates

As noted in #114 it seems like putting an include that dumps html on a line with content can result in escaped html.

Cases where this occurs seem unintentional and we may want to catch and warn about them automatically, at least.

Note that whitespace-ignoring includes can actually make things worse in this, e.g.

```
here is some text {%- include my include -%}

## Here is some markdown
```

Will result in a rendering that also escapes the later markdown, i.e.:
```
here is some text ## Here is some markdown
```

No file, test, or entry point is named. Start by locating the repository's existing include or template validation, then reproduce the inline include example and define a warning for cases that cause later HTML or Markdown to be escaped. Done means the problematic pattern is detected automatically without changing valid whitespace-ignoring includes.

Written by the indexing model from the issue text.