Replace loadGeometry() with a geometry getter

[jfirebaugh]

Since we're talking about a major version bump for #33, I'd like to propose this API change as well. Replacing loadGeometry() with a geometry property will make it easier, for testing purposes, to substitute a plain old JS object where a VectorTileFeature would normally be used.

In addition, we have the option of caching the generated geometry internally if we later think that makes sense, without changing the API or ending up with a misleading method name.

[mourner]

I think we should add caching if we want to make this change, since otherwise it would be easy to misuse — people don't expect a simple property access to be very computationally intensive (in this case, reparsing the whole geometry from scratch), and loadGeometry() indicates better that something's going on under the hood.

I'm divided on this change generally. And it's still as easy to substitute on the testing side:

feature.loadGeometry = function () { return mockGeometry; };

[mourner]

@jfirebaugh with VT spec 2.0 released, let's move on this PR. Two options to go forward:

1. Leave feature.geometry as a getter but cache it after parsing so that it doesn't get reparsed on every access.
2. Revert to public loadGeometry() method.

I'm fine with either but prefer the latter option since it's more explicit.

[mourner]

Stale.