template.html:
<H>%%insert_here%%</H>
sldfjkdslkf
sdlfkjsdlf
sldkjfdslk
<B>%%insert_here%%</B>

data.txt:
hello

foo.html:
<H>hello
</H>
sldfjkdslkf
sdlfkjsdlf
sldkjfdslk
<B>hello
</B>