It seems that some time in the last couple years wkhtmltopdf's default
margins were changed from '10mm' to zero. As an alternative to #177,
this PR adds an option to pass in arbitrary wkhtmltopdf long arguments
and sets the default top and bottom margin to '10mm'.
Merge implicit and explicit generation paths, start emitting file
transform & copy signals, fix various bugs, introduce new bugs, support
better --debug outputs in the future.
Remove file-based open methods from resume classes; force clients to use
clean string-based or JSON overloads; fix processing glitch in
validate(); tweak outputs; adjust tests; update CHANGELOG; etc.
