Convert global 'this' references to 'global' before transpiling

[sgtcoolguy]

This adds a very simple babel plugin to convert global this references to global references in advance of transpilation (which would otherwise convert them to undefined). There's a unit test added to confirm the fix.

Relates to:

• Replace underscore with lodash, update backbone to 1.3.3 alloy#842 (replacing underscore with lodash. underscore is the library with global this reference)
• [TIMOB-24404] Expose global object on iOS titanium-sdk#9552 (adding global as a global!)
• [TIMOB-24610] Support transpiling user JS code to ES5, or minimum target OS/JS engine support titanium-sdk#9512 (hooking transpiling to SDK builds)
• [TIMOB-24610] Implement ability to transpile #20 (moving transpiling to this package)
• [TIMOB-25650] Upgrade to babel 7 ecosystem #23 and [TIMOB-25768] Implement ability to use rollup #24 - moves us to babel 7, at least in part to avoid this issue.

[ewanharris]

LGTM

Built and ran through KitchenSink-v2 on Android, iOS and Windows, runs as expected. No problems seen and the app functions as expected

Would like to note that it appears that Uncaught TypeError: Cannot read property '_' of undefined never actually occurred on Windows as I had no issues running KS on Windows with 7.1.0 and transpile enabled before this change.

[garymathews]

CR: PASS