Add xmllint linter (#559)

* Add xmllint linter for xml

doc/ale-xml.txt

@@ -0,0 +1,26 @@
+===============================================================================
+ALE XML Integration                                         *ale-xml-options*
+
+
+-------------------------------------------------------------------------------
+xmllint                                                    *ale-xml-xmllint*
+
+g:ale_xml_xmllint_executable                   *g:ale_xml_xmllint_executable*
+                                               *b:ale_xml_xmllint_executable*
+  Type: |String|
+  Default: `'xmllint'`
+
+  This variable can be set to change the path to xmllint.
+
+
+g:ale_xml_xmllint_options                         *g:ale_xml_xmllint_options*
+                                                  *b:ale_xml_xmllint_options*
+  Type: |String|
+  Default: `''`
+
+  This variable can be set to pass additional options to xmllint.
+
+
+-------------------------------------------------------------------------------
+  vim:tw=78:ts=2:sts=2:sw=2:ft=help:norl:
+

doc/ale.txt

@@ -85,6 +85,8 @@ CONTENTS                                                         *ale-contents*
       tslint..............................|ale-typescript-tslint|
     vim...................................|ale-vim-options|
       vint................................|ale-vim-vint|
+    xml...................................|ale-xml-options|
+      xmllint.............................|ale-xml-xmllint|
     yaml..................................|ale-yaml-options|
       yamllint............................|ale-yaml-yamllint|
   5. Commands/Keybinds....................|ale-commands|
@@ -177,6 +179,7 @@ The following languages and tools are supported.
 * Vim: 'vint'
 * Vim help: 'proselint'
 * XHTML: 'proselint'
+* XML: 'xmllint'
 * YAML: 'yamllint'
 
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