Encode invalid XML characters instead of skipping them.

The <sp> element in <programlisting> has a new optional attribute
`value`, which contains value of given invalid ASCII character. In
case of space, the `value` attribute is omitted.

Use cases for this: including snippets of *very esoteric* languages,
markup that makes use of the advanced ASCII formatting characters or
for example highlighting a console output containing ANSI color codes
(which is my case, in fact).

Regarding backwards compatibility -- as files with such ASCII
characters are very rare, I don't expect this minor difference in the
output to be a problem. Besides that, such ASCII characters are often
replaced by a space in many applications anyway.

A test snippet was extended to contain a special character so this
difference in behavior could be verified.

1 files changed, 2 insertions, 0 deletions

diff --git a/testing/example_test.cpp b/testing/example_test.cpp
index f589023..a117b63 100644
--- a/testing/example_test.cpp
+++ b/testing/example_test.cpp
@@ -1,5 +1,7 @@
 void main()
 {
+  const char* a = "Some special character here:  ";
+
   Test t;
   t.example();
 }