Align when {{#include}} is indented

[kynthus]

Fixes #1564
Considering the impact on embedding simple code, it may be necessary to discuss whether to make it optional or introduce it, so I made it a draft.

[kynthus]

I stopped using trim(), which destroys whitespace at the begin and end of files.
And unwrap_or_else() is useless lazy evaluation, I use unwrap_or() instead.

[tshepang]

works for me... had to rebase on master to avoid "Error: The "mermaid" preprocessor exited unsuccessfully"