If the markdown contains something that is indented by more that the
`leadingTabs`/`leadingWs` then extra white space is incorrectly removed.
ie the following example:
```
<section data-markdown>
some text
indented text
more indented text
</section>
```
would result in the following markdown:
```
some text
indented text
more indented text
```
We can work around this problem by using a function to generate the
replace value.