Added ability to run on multiple files at once

[voletiv]

No description provided.

[severin-lemaignan]

@voletiv thank you for your contribution. I'm happy to merge this feature, but instead of creating a new gazr_benchmark_head_pose_multiple_frames binary which is almost the same as the 'single file' one, could you instead add a command-line option to the 'single file' version to accept a list of files to process?

Something like:

./gazr_benchmark_head_pose --files frames.txt ../share/shape_predictor_68_face_landmarks.dat

[voletiv]

@severin-lemaignan Done! Please check.

[severin-lemaignan]

Great. Can you squash your 3 commits into a single one, and then, I'll merge your PR.

[voletiv]

Squashed, please check.

[severin-lemaignan]

Merged, thanks!