Add properties syntax highlighting for docs

Fixes gh-495

spring-boot-docs/src/main/docbook/xsl/xslthl-config.xml

@@ -16,5 +16,6 @@
 	<highlighter id="css" file="./xslthl/css-hl.xml" />
 	<highlighter id="sql" file="./xslthl/sql2003-hl.xml" />
 	<highlighter id="asciidoc" file="./xslthl/asciidoc-hl.xml" />
+	<highlighter id="properties" file="./xslthl/properties-hl.xml" />
 	<namespace prefix="xslthl" uri="http://xslthl.sf.net" />
 </xslthl-config>

spring-boot-docs/src/main/docbook/xsl/xslthl/properties-hl.xml

@@ -0,0 +1,38 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!--
+
+Syntax highlighting definition for Java
+
+xslthl - XSLT Syntax Highlighting
+http://sourceforge.net/projects/xslthl/
+Copyright (C) 2005-2008 Michal Molhanec, Jirka Kosek, Michiel Hendriks
+
+This software is provided 'as-is', without any express or implied
+warranty.  In no event will the authors be held liable for any damages
+arising from the use of this software.
+
+Permission is granted to anyone to use this software for any purpose,
+including commercial applications, and to alter it and redistribute it
+freely, subject to the following restrictions:
+
+1. The origin of this software must not be misrepresented; you must not
+   claim that you wrote the original software. If you use this software
+   in a product, an acknowledgment in the product documentation would be
+   appreciated but is not required.
+2. Altered source versions must be plainly marked as such, and must not be
+   misrepresented as being the original software.
+3. This notice may not be removed or altered from any source distribution.
+
+Michal Molhanec <mol1111 at users.sourceforge.net>
+Jirka Kosek <kosek at users.sourceforge.net>
+Michiel Hendriks <elmuerte at users.sourceforge.net>
+
+-->
+<highlighters>
+	<highlighter type="oneline-comment">#</highlighter>
+	<highlighter type="regex">
+		<pattern>^(.+)(?==|:)</pattern>
+		<style>attribute</style>
+		<flags>MULTILINE</flags>
+	</highlighter>
+</highlighters>