Flags for font name/package name are not consistent with README

[mhernz]

Per the README:

Flutter class options:
- `-p` or `--package=<name>`
Name of a package that provides a font. Used to provide a font through package dependency.

Font options:
- `-f` or `--font-name=<name>`
Name for a generated font.

However, in cli/options.dart the actual flags for these options are -f/--package and -n/--font-name.

In my opinion -f for --package is too confusing, and it is worth introducing a breaking change to rename this flag to -p.