Add optional parameter 'market' to getTrack and getTracks

[lynnpark]

getTrack and getTracks have an optional parameter 'market'. This is needed in order to get a non-null value for the preview_url field.

[lynnpark]

I can't tell if this is still maintained. Are PRs being accepted?

[JMPerez]

@lynnpark Unfortunately I have very little time these days to get involved in this project. There is an open issue asking for new maintainers on #114

[JMPerez]

Closing the issue. market can be passed in the options parameter got both getTrack() and getTracks().