Add custom context vendor configuration option

[alexanderdean]

This should be in the tracker constructor.

If an event has custom context and custom context vendor is set, then attach &cv=[custom context vendor] to the payload.

/cc Yali (@yalisassoon)

[alexanderdean]

See also snowplow/snowplow-javascript-tracker#180

[alexanderdean]

Moved back to 0.3.0, thanks Fred.

[alexanderdean]

Done, closing.