Add iOS prebuild

[vweevers]

Forked from #575. Exact details TBD. At the very least, we need to:

• Have people that are qualified to review and test
• Decide whether to use the nodejs-mobile(-gyp) forks or node core. I prefer the latter because I don't want us to get stuck with nodejs-mobile. Someone must step up and champion it. Node 12 updated to V8 7.4 which introduced an interpreter-only mode, so in theory node can support iOS now (to my limited understanding).
• Add a separate Travis job (using OSX?)
  • If nodejs-mobile: set PREBUILD_NODE_GYP=nodejs-mobile-gyp
• Find out what the value of os.platform() is; node-gyp-build must be able to detect iOS.

[vweevers]

Removing this from the backlog because there's no actionable item for us. As noted above we need external help.

[vweevers]

Can reopen if there's interest and someone to champion it.