Add node_modules support for JSHint, and use the global config as a fallback.

doc/ale.txt

@@ -399,8 +399,25 @@ g:ale_javascript_jshint_executable         *g:ale_javascript_jshint_executable*
   Type: |String|
   Default: `'jshint'`
 
+  ALE will first discover the jshint path in an ancestor node_modules
+  directory. If no such path exists, this variable will be used instead.
+
   This variable can be changed to change the path to jshint.
 
+  If you wish to use only a globally installed version of jshint, set
+  |g:ale_javascript_jshint_use_global| to `1`.
+
+
+g:ale_javascript_jshint_use_global         *g:ale_javascript_jshint_use_global*
+
+  Type: |String|
+  Default: `0`
+
+  This variable controls whether or not ALE will search for a local path for
+  jshint first. If this variable is set to `1`, then ALE will always use the
+  global version of jshint, in preference to locally installed versions of
+  jshint in node_modules.
+
 
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 4.3. phpcs                                           *ale-linter-options-phpcs*