Flexible requirements (allow for opencv-python or opencv-python-headless)

[Erotemic]

I was looking through the code and noticed that the setup.py was using a the parse_requirements function that I wrote some time back. I also noticed there was an explicit dependency on opencv-python. This causes issues for reasons that I outline in an mmdet PR: open-mmlab/mmcv#2775

I've made a few changes / upgrades:

• moved requirements.txt -> requirements/runtime.txt
• Added requirements/cv2.txt. and requirements/cv2-headless.txt
• Added a new requirements.txt that references requirements/runtime.txt and requirements/cv2.txt (this means that pip install -r requirements.txt works exactly as before).
• Changed setup.py to depend only on requirements/runtime.txt, this means that to get a full install, the user will have to specify pip install -e .[cv2] xor pip install -e .[cv2-headless], which unfortunately is unavoidable if you want to support the ability to use either version of the cv2 package. This is a minor breaking change.
• Added minimum versions to all requirements
• Added the ability to pip install -e .[runtime-strict,cv2-strict], which will install the package with dependencies pinned to the minimum version (very useful for testing / ensuring upstream package changes don't break your code).

In summary this adds:

• Feature: Gives the user flexibility to use either opencv-python or opencv-python-headless
• Feature: Gives the user the ability to install requirements with "strict" versions, where dependencies are pinned to minimum versions
• Minor Breaking Change: Means that you can no longer pip install -e . to install all dependencies you have to pip install -e .[cv2] xor pip install -e .[cv2-headless] or preinstall the opencv package you want.

Pending review additional work that may need to be done:

• Update any install scripts to choose which version of cv2 to use.
• Check if the chosen minimum versions are reasonable or if they need to change.