Add integration mixpanel

This commit copies the content of the integration repo into
the "integrations" folder.

Original repo: https://github.com/segment-integrations/analytics.js-integration-mixpanel
Readme: https://github.com/segment-integrations/analytics.js-integration-mixpanel/blob/master/README.md

Author: SegmentDestinationsBot

integrations/mixpanel/HISTORY.md

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+3.0.1 / 2018-07-18
+==================
+  * Supports mapping to settings.sourceName on `track` and `page` calls (#32)
+  * Fix flaky tests (#31)
+
+3.0.0 / 2017-10-19
+==================
+
+  * Parity rewrite (#29)
+  * Bump a.js-int-tester version to ^3.1.0 (#28)
+  * Pin karma, karma-mocha dev dependencies (#27)
+
+2.2.0 / 2016-08-16
+==================
+
+  * update snippet
+
+2.1.0 / 2016-08-12
+==================
+
+  * tag mp_lib with Segment: web
+
+2.0.0 / 2016-06-21
+==================
+
+  * Remove Duo compatibility
+  * Add CI setup (coverage, linting, cross-browser compatibility, etc.)
+  * Update eslint configuration
+
+
+1.5.0 / 2016-05-12
+==================
+
+  * Update Page Call Logic with consolidatedPageCalls setting
+
+1.4.2 / 2016-05-07
+==================
+
+  * Bump Analytics.js core, tester, integration to use Facade 2.x
+
+1.4.1 / 2016-02-03
+==================
+
+  * fix multiple page call issue
+
+1.4.0 / 2015-12-09
+==================
+
+  * add persistence config
+
+1.3.2 / 2015-11-03
+==================
+
+  * make mixpanel es5 friendly
+  * Dont set peopleProperties in track calls.
+
+1.3.1 / 2015-10-29
+==================
+
+  * Dont set peopleProperties in track calls. Dont map special traits in track, people should use identify instead.
+
+1.3.0 / 2015-10-27
+==================
+
+
+
+1.3.0 / 2015-10-27
+==================
+
+  * refactor super people props
+
+1.2.1 / 2015-09-08
+==================
+
+  * adding check for no Mixpanel.superProperties object closes #11
+
+1.2.0 / 2015-09-08
+==================
+
+  * Allow explicit declaration of people/super properties ([More Info](https://github.com/segment-integrations/analytics.js-integration-mixpanel/pull/10))
+  * stub every possible mixpanel method
+
+1.1.0 / 2015-08-25
+==================
+
+  * Merge pull request #6 from segment-integrations/people-super-properties
+  * added enhancedTrack support
+
+1.0.5 / 2015-08-05
+==================
+
+  * Send arrays through to Mixpanel
+
+1.0.4 / 2015-06-30
+==================
+
+  * Replace analytics.js dependency with analytics.js-core
+
+1.0.3 / 2015-06-24
+==================
+
+  * Bump analytics.js-integration version
+
+1.0.2 / 2015-06-24
+==================
+
+  * Bump analytics.js-integration version
+
+1.0.1 / 2015-06-10
+==================
+
+  * Bump dependency versions
+
+1.0.0 / 2015-06-09
+==================
+
+  * Initial commit :sparkles:

integrations/mixpanel/README.md

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+# analytics.js-integration-mixpanel [![Build Status][ci-badge]][ci-link]
+
+Mixpanel integration for [Analytics.js][].
+
+## License
+
+Released under the [MIT license](LICENSE).
+
+
+[Analytics.js]: https://segment.com/docs/libraries/analytics.js/
+[ci-link]: https://circleci.com/gh/segment-integrations/analytics.js-integration-mixpanel
+[ci-badge]: https://circleci.com/gh/segment-integrations/analytics.js-integration-mixpanel.svg?style=svg