Add integration with buffalo

[stnguyen90]

fixes #194

[rhcarvalho]

Please have a look at the Travis builds https://github.com/getsentry/sentry-go/pull/196/checks?check_run_id=572336394

The current code doesn't compile.

[rhcarvalho]

Adding this will pull in a good bunch of new deps into go.mod, which is unfortunate.

Would be good to advance on #156 before, so that we don't bloat users' go.mod when they don't need the integration.

Alternatively, I'm also tracking golang/go#36460 which might alleviate the problem and allow us to have a single module == easier maintenance and distribution/consumption -- single coherent version. That's planned for Go 1.15.

[github-actions]

This pull request has gone three weeks without activity. In another week, I will close it.

But! If you comment or otherwise update it, I will reset the clock, and if you label it Status: Backlog or Status: In Progress, I will leave it alone ... forever!

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"A weed is but an unloved flower." ― Ella Wheeler Wilcox 🥀