Have `sgml-attribute-offset' control SGML attribute indentation

Fixes: debbugs:20161

* textmodes/sgml-mode.el (sgml-attribute-offset): New defcustom.
(sgml-calculate-indent): Use `sgml-attribute-offset' for attribute
indentation.

lisp/ChangeLog

@@ -1,3 +1,9 @@
+2015-03-22  Jackson Ray Hamilton  <jackson@jacksonrayhamilton.com>
+
+	* textmodes/sgml-mode.el (sgml-attribute-offset): New defcustom.
+	(sgml-calculate-indent): Use `sgml-attribute-offset' for attribute
+	indentation (bug#20161).
+
 2015-03-22  Dmitry Gutov  <dgutov@yandex.ru>
 
 	* json.el (json-decode-char0): Delete this alias.

lisp/textmodes/sgml-mode.el

@@ -46,6 +46,25 @@
   :type 'integer
   :group 'sgml)
 
+(defcustom sgml-attribute-offset 0
+  "Specifies a delta for attribute indentation in `sgml-indent-line'.
+
+When 0, attribute indentation looks like this:
+
+  <element
+    attribute=\"value\">
+  </element>
+
+When 2, attribute indentation looks like this:
+
+  <element
+      attribute=\"value\">
+  </element>"
+  :version "25.1"
+  :type 'integer
+  :safe 'integerp
+  :group 'sgml)
+
 (defcustom sgml-xml-mode nil
   "When non-nil, tag insertion functions will be XML-compliant.
 It is set to be buffer-local when the file has
@@ -1510,13 +1529,13 @@ LCON is the lexical context, if any."
     (`pi nil)
 
     (`tag
-     (goto-char (1+ (cdr lcon)))
+     (goto-char (+ (cdr lcon) sgml-attribute-offset))
      (skip-chars-forward "^ \t\n")	;Skip tag name.
      (skip-chars-forward " \t")
      (if (not (eolp))
 	 (current-column)
        ;; This is the first attribute: indent.
-       (goto-char (1+ (cdr lcon)))
+       (goto-char (+ (cdr lcon) sgml-attribute-offset))
        (+ (current-column) sgml-basic-offset)))
 
     (`text

test/indent/sgml-mode-attribute.html

@@ -0,0 +1,14 @@
+<element attribute="value"></element>
+
+<element
+    attribute="value">
+  <element
+      attribute="value">
+  </element>
+</element>
+
+<!--
+    Local Variables:
+    sgml-attribute-offset: 2
+    End:
+  -->