Show filenames in output dictionaries

[davelab6]

• [x] add a Filename key to the Font dictionary, so that instead of e.g.

$ pyfontaine --collection subsets builds/d088571/sources/RhodiumLibre-Regular.ttf;
Fonts
  Font:
    Common name: Rhodium Libre
    Sub family: Regular
    Weight: normal
    Fixed width: no
    Fixed sizes: no
...etc...

I see

Fonts
  Font:
    Filename: RhodiumLibre-Regular.ttf
    Common name: Rhodium Libre
    Sub family: Regular
    Weight: normal
    Fixed width: no
    Fixed sizes: no
...etc...