Feature/typescript typings

[dawnmist]

• Update captureException to match support of ErrorEvent and string as permitted inputs for the exception (fixes issue Incorrect TypeScript typings #1167).
• Update setExtraContext to allow using setExtraContext() to clear the extra data.
• Update setTagsContext to allow using setTagsContext() to clear the tags data.
• Add tests for the above changes.
• Add additional test calls for the Raven.context and Raven.wrap to take a function that throws an ErrorEvent.
• Update minimum Typescript version to 2.3.0, as the type definitions for ErrorEvent are incorrect/unusable in versions of Typescript less than 2.3.0.

Before submitting a pull request, please verify the following:

• If you've added code that should be tested, please add tests.
• If you've modified the API (e.g. added a new config or public method), update the docs and TypeScript declaration file
• Ensure your code lints and the test suite passes (npm test) - the changes are solely to the typescript definitions as expansions of the current function definitions. The typescript tests work, but there are a few breadcrumb tests that are failing on Firefox 52.0.0/Linux. I believe those failures are completely unrelated to the typescript function type definitions, and thus not relevant to this pull request.

[kamilogorek]

Thanks @dawnmist! :)