font_project: customize variable and ttf-interpolatable output path
font_project: customize variable and ttf-interpolatable output path

using either output_path or output_dir options

notable change is that when doing "-o variable", the
master_ttf_interpolatable are no longer created in the current directory
but only in a temporary folder.
Unless one passes both -o "ttf-interpolatable variable" formats, in
which case the result of one is used as input for the other.

Incidentally this also fixes https://github.com/googlei18n/fontmake/issues/419
because I removed an unnecessary call to `_designspace_locations` method
inside `build_variable_font`. So one can now build a variable font
without any instances' filenames in the designspace.

googlei18n/fontmake,googlefonts/fontmake,googlefonts/fontmake,googlei18n/fontmake