New option --get-filename to print output filename

New option --get-filename to print output filename

When using youtube-dl within scripts, it is very useful to know
what will be the final output filename with all the title settings and
filename templates applied. Add option to the quiet mode operations
to print that info.

For this I had to move the filename-generation into its own function.
As much as I can tell it should work almost always well, ie. not to
break things if one not actually interested in the title, like in
case of other forced printing. That is, unless there's an invalid
system charset or the user specified a wrong output template. In that
case probably could be assumed that the user does have a problem
(the former) or did want to mess with the filename (the latter).

Signed-off-by: Gergely Imreh <imrehg@gmail.com>