FUNCT Add sel_string:format/2
FUNCT check after testing

Most of the errors are caught while testing, and checking will become slow when
we add dialyzer
FUNCT use simple expansion by default in the Makefile

It is usually the fastest and most controllable way of assignment, even though
in this case affects only the REBAR variable
FUNCT create the documentation as part of the target all
FUNCT split xref to its own target

We will be adding dialyzer to the checks as well
FUNCT run dialyzer as part of the check target
BUGFIX make dialyzer happy

I am not sure that format_entry was broken anyway, but not passing the list
through a final io_lib:format upsets dialyzer
BUGFIX trivial fixes to avoid dialyzer warnings
DOC mention dialyzer as static check