[smerl.erl] Add options support (src_dir)
[smerl.erl] Add options support (src_dir)

Manually merge
https://github.com/khia/erlyweb/commit/cafe315eb39cde2ce13403868b2b37e6ac355454,
pulling out a helper function `do_get_extend_data/4` to avoid nesting too
deeply.

See #1

deadtrickster/smerl