Add support for loading and saving OBJ files

[codecov[bot]]

Codecov Report

Attention: 1 lines in your changes are missing coverage. Please review.

Comparison is base (39facbb) 94.57% compared to head (43c60cd) 94.74%.

Files	Patch %	Lines
src/extra/obj.jl	97.43%	1 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #57 +/- ## ========================================== + Coverage 94.57% 94.74% +0.17% ========================================== Files 14 15 +1 Lines 737 780 +43 ========================================== + Hits 697 739 +42 - Misses 40 41 +1 ```

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