Breaking: Use browserslist defaults

Use browserslist defaults instead of our own when none are specified.
Update the documentation related to this property as well as the tests
of rules that use it.

- - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - - -

Fix #452

Close #453
Close #451

docs/user-guide/index.md

@@ -196,8 +196,8 @@ your `.sonarrc` file.
 ## Browser configuration
 
 `sonar` allows you to define your browser support matrix by adding
-the property `browserlist` to your `.sonarrc` file. This property
-follows the same convention as [`browserlist`][browserslist]:
+the property `browserslist` to your `.sonarrc` file. This property
+follows the same convention as [`browserslist`][browserslist]:
 
 ```json
 {
@@ -217,6 +217,17 @@ versions of IE prior IE10, or tell you to remove that tag or header
 it you only need IE11+ because document modes were removed at that
 version.
 
+If no value is defined, [`browserslist`'s defaults][browserslist defaults] will
+be used:
+
+```js
+browserslist.defaults = [
+    '> 1%',
+    'last 2 versions',
+    'Firefox ESR'
+];
+```
+
 ## Connectors
 
 A `connector` is the interface between the `rule`s and the website
@@ -262,7 +273,8 @@ to know more.
 
 <!-- Link labels: -->
 
-[browserlist]: https://github.com/ai/browserslist
+[browserslist]: https://github.com/ai/browserslist
+[browserslist defaults]: https://github.com/ai/browserslist/blob/3b8e4abfbfe36d01859a0e70292106be0fe70c8f/index.js#L303
 [homebrew]: https://brew.sh/
 [nodejs]: https://nodejs.org/en/download/current/
 [npm change default directory]: https://docs.npmjs.com/getting-started/fixing-npm-permissions#option-2-change-npms-default-directory-to-another-directory