Add new tool and article.

Add the batch replace tool. Syntax

flx_batch_replace dir srcregex dstrepl regex repl

which searches dir for files matching srcregex,
replaces regex with repl in the file's text,
and write dstrepl. The replacement patterns use \9 notation
where 9 is the subgroup number. File replacement uses the
same mechanism as flx_cp to prevent clobbering any files.