feat: pull country or language by code.

* feat: pull country or language by code.

This differs from the existing `search` functionality in that it allows
you to explicitly pull back a single country or language instead of
anything that matches (which could result in more items returned than
expected).

* refactor(types): add new types for the affected endpoints.
* docs(readme): update to include examples for new functionality.
* test: expand tests to include new parameters.
* feat(package): upgrade dependencies to the latest versions.
* fix(eslint): adjust configuration based on an updated dependency.
* docs(license): bump the year of the license.

README.md

@@ -103,7 +103,15 @@ holidayApi.holidays({ country: 'US', year: 2019 })
 holidayApi.countries();
 ```
 
-#### Search for a country by code or name
+#### Fetch a supported country by code
+
+```javascript
+holidayApi.countries({
+  country: 'NO',
+});
+```
+
+#### Search for countries by code or name
 
 ```javascript
 holidayApi.countries({
@@ -119,7 +127,15 @@ holidayApi.countries({
 holidayApi.languages();
 ```
 
-#### Search for a language by code or name
+#### Fetch a supported language by code
+
+```javascript
+holidayApi.language({
+  language: 'es',
+});
+```
+
+#### Search for languages by code or name
 
 ```javascript
 holidayApi.language({