Correct variable font and loading font via cmd (#14)
New feature:
- Convert legacy weight into "new" weight.
Corrected bugs:
- Families and fullname would use the same set.
- In very specific case, matplotlib.findSystemFonts can return non Font file.
- The font would be considered like a variable font if it contain an fvar table. That was false, it need to contain an fvar table AND a stat table to be considered like a variable font (resolve Option --additional-fonts doesn't work #13)
- Correct overloading fonts with --additional-fonts (resolve Option --additional-fonts doesn't work #13)
