[facebook-pixel] add support for trackSingle

[SegmentDestinationsBot]

Migrated from #38 by @jaredmcdonald

it would be great to add support for making requests for specific pixels with fbq('trackSingle', 'PIXEL_ID', ...) to disambiguate a Segment-managed pixel from other pixels that might be on a page

see https://developers.facebook.com/ads/blog/post/2017/11/28/event-tracking-with-multiple-pixels-tracksingle/

[johnnykramer]

we also need it!

[ajlai]

Also need it! Otherwise we'll need to go through Google Tag Manager in order to set up trackSingle

[doctor-spaceman]

I would also love to see trackSingle support for the Facebook destination.

[benmartin9]

Also need it, please!
Key feature, we'll have to move to GTM otherwise.