Even in relatively small projects using `--trace` (and `--trace-expand`)
may produce a lot of output. When developing a custom module usually
one is interested in output of only a few particular modules.
Add a `--trace-source=<file>` option to enable tracing only a subset of
source files. The final output would be only from requested modules,
ignoring anything else not matched to given filename(s).
Alex Turbov