Add support cancellation token for Task

[pddkhanh]

Hi team,

First of all, thank you for your great work. I've used Bolts-ObjC and now migrating to BoltsSwift, I used BFCancellationToken on the old project, so I've tried to implement the CancellationToken for this Swift version. Please help to review it and suggest me to fix or improve it. This is the first time I create the Pull Request. So please patient if I missed somethings.

Thanks

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[serejahh]

Hi @nlutsenko! Could you check the PR? We'd really like to have the feature :)

[gapl]

Are there any plans to merge this in?

[facebook-github-bot]

@itleo31 I tried to find reviewers for this pull request and wanted to ping them to take another look. However, based on the blame information for the files in this pull request I couldn't find any reviewers. This sometimes happens when the files in the pull request are new or don't exist on master anymore. Is this pull request still relevant? If yes could you please rebase? In case you know who has context on this code feel free to mention them in a comment (one person is fine). Thanks for reading and hope you will continue contributing to the project.