Support multiple group name

[simnalamburt]

obj spec supports multiple group name since this project's current implementation of obj-rs doesn't.

g group_name1 group_name2 . . .

Polygonal and free-form geometry statement.

Specifies the group name for the elements that follow it. You can
have multiple group names. If there are multiple groups on one
line, the data that follows belong to all groups. Group information
is optional.

group_name is the name for the group. Letters, numbers, and
combinations of letters and numbers are accepted for group names.
The default group name is default.