Add option for a promise based externalGeocoder

[mapbox-danny]

Proposal to add an option for an externalGeocoder, designed to accept a promise resolving to an array of features. This Update would allow a server-side api/resource to be called asynchronously.

• briefly describe the changes in this PR
• write tests for all new functionality
• run npm run docs and commit changes to API.md
• update CHANGELOG.md with changes under master heading before merging

[andrewharvey]

hi @mapbox-danny did you see #165 (comment) where we discussed leaving localGeocoder as is and creating an additional externalGeocoder which supports a promise. The idea being localGeocoder then works with local sources you already have loaded, and externalGeocoder where you have an external API to use.

[mapbox-danny]

@andrewharvey, your feedback to use externalGeocoder to add support for promises makes a lot of sense. I have updated the proposed approach to keep localGeocoder effectively unchanged and to add in the option for the externalGeocoder.

[andrewharvey]

Oh wow you even managed to fix the broken unit tests unrelated to this feature. Thanks!

I haven't done a deep look at all the changes, but overall it looks good.

[mapbox-danny]

Oh wow you even managed to fix the broken unit tests unrelated to this feature. Thanks!

I haven't done a deep look at all the changes, but overall it looks good.

Thanks @andrewharvey. Let me know what you think about getting it ready to merge when you have a chance.

[andrewharvey]

Looks good to me thanks. I haven't done detailed testing, but happy with the testing you've done.