Opencv4nodejs: Support for BFMatcher Constructor for crossCheck=True

[eek]

Hi there!

Is there a way to easily enable crossCheck for Brute Force Feature Matcher?

```# create BFMatcher object
bf = cv2.BFMatcher(cv2.NORM_HAMMING, crossCheck=True)

Match descriptors.

matches = bf.match(des1,des2)
```

Currently Using the SURF Detector and would be great to have the crossCheck function.

[justadudewhohacks]

Descriptor matching is currently implemented here. Maybe there is an easy way to implement the flag as a parameter. In the worst case the Matcher classes would have to be implemented and exposed.

[justadudewhohacks]

Your changes have been published to npm with v4.8.0.