Show progress as we transcode and copy files

We use ANSI codes to overwrite a previous "in progress" message
with a message stating that the operation is finished. A floating
line, such as a progress line, is displayed below all non-floating
lines, and can be "unfloated", i.e., turned into a regular printed
line that doesn't float below anything.
Call fflush after printf/fprintf

Looks like \n in printf/fprintf only causes a fflush if stdio/fp
is connected to a terminal.
Don't print floating messages writing to pipes/files

Prevents ANSI control codes showing up in files.
Show individual progress per file as percentage
Buffer stdout to avoid flickering
Updated docs to reflect recent changes