--replace-command remove spurious white line

[tdegeus]

Consider

This is
\detail{
    ...
}
a line.

Then running

texcleanup --replace-command "{\detail}[1]" "" main.tex

should result in

This is
a line.

and not (as currently) in

This is

a line.

[tdegeus]

It could be good to merge _replace_command_impl and _find_args into a single function at this time