Expose methods to define new proj4 definitions

[jbeezley]

This exposes proj4.defs as geo.transform.defs and also adds a method to lookup proj4 definitions from epsg.io. This can be used as follows:

geo.transform.lookup('EPSG:26986').then(function () {
    // Now the projection can be used
    geo.transform.transformCoordinates('EPSG:26986', 'EPSG:4326', {x: 0, y: 0});
});

[aashish24]

this is nice @jbeezley. @mgrauer I know you don't need it but could you try it with your previous commit?

[mgrauer]

I'll check it out, possibly tomorrow afternoon.

[aashish24]

LGTM 👍

[codecov-io]

Current coverage is 70.70%

Merging #561 into master will increase coverage by +<.01%

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  Files            81         81          
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  Methods           0          0          
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+ Hits           4975       4989    +14   
- Misses         2066       2068     +2   
  Partials          0          0          

1. File src/mapInteractor.js (not in diff) was modified. more
   • Misses +2
   • Partials 0
   • Hits -2

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