rename "Words" to "Fields" everywhere

Because "Words" is about to obtain a new meaning.
also change word to field in the doc
Distinguish separators from delimitors

We are about to add a new kind of separator which is not based on a
delimiter string.
separate fields by Whitespace by default

No need for the alternate "Words" constructor for LineFormat after all.
Instead, we have the alternate "Whitespace" constructor for Separator.
remove extraneous space
"field" instead of "word" in README
rename "line" to "record" everywhere
Merge branch 'issue-119', closes #119