Get & set options

[scottsfarley93]

This PR fixes #176 by adding getter/setter methods.

This PRs get/set methods for the following options:

• language
• zoom
• flyTo
• placeholder
• bbox
• countries
• types
• minLength
• limit
• filter
• proximity (already implemented)

It's an open question as to whether we should support get, and particularly set, methods for these options, since changing their behavior more fundamentally alters the behavior of the plugin -- and I'm not sure of the use case of changing them after creating an instance with different values:

• accessToken
• origin
• localGeocoder
• reverseMode
• reverseGeocode

#208 will add get/set functions for html rendering functions and we should add analogous methods in #219 to support new options that we're adding in version 4 (#197)

• 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

\cc @katydecorah @yuletide

[katydecorah]

This brings sooo much customization 😍

I hear you on wanting to be consistent on these get and set options, but I think it's also fair to not add options for the few that you listed your PR comment. It's hard to think of valuable use cases for them. Nevertheless, not that we have these patterns in place it should be fairly straightforward to add if someone requests a getter/setter that we don't already have.

I left you several comments with some (nitpicky) documentation suggestions, please take or leave as you see fit.

[katydecorah]

Small copy suggestion:

Suggested change

	* @param {Number} zoom On geocoded result what zoom level should the map animate to when a `bbox` isn't found in the response. If a `bbox` is found the map will fit to the `bbox`.
	* @param {Number} zoom The zoom level that the map should animate to when a `bbox` isn't found in the response. If a `bbox` is found the map will fit to the `bbox`.

[katydecorah]

Small edit for consistency:

Suggested change

	* Sets the flyTo options
	* Set the flyTo options

[katydecorah]

For consistency:

Suggested change

	* Gets the bounding box used by the plugin
	* Get the bounding box used by the plugin

[katydecorah]

Suggested change

	* Sets the bounding box to limit search results to
	* Set the bounding box to limit search results to

[katydecorah]

Should this be types?

Suggested change

	* Set the countries to limit search results to
	* Set the types to limit search results to

[katydecorah]

Suggested change

	* Get the limit value used by the plugin
	* Get the limit value for the number of results to display used by the plugin

[katydecorah]

Maybe call out what the minimum length refers to?

Suggested change

	* Get the minimum length used in the plugin
	* Get the minimum number of characters typed to trigger results used in the plugin

[katydecorah]

Suggested change

	* Set the minimum length value used by the plugin
	* Set the minimum number of characters typed to trigger results used by the plugin

[katydecorah]

Suggested change

	* Set the limit value used by the plugin
	* Set the limit value for the number of results to display used by the plugin

[katydecorah]

I'm not exactly sure what filter does in suggestions, maybe we could add a little more to add context?

[andrewharvey]

It filters the results shows 😉 See https://docs.mapbox.com/mapbox-gl-js/example/mapbox-gl-geocoder-limit-region/