Add real handling for preserve indent.

Now that I know which flag changes this setting, we can handle it properly.

src/types/lesser_element.rs

@@ -50,6 +50,7 @@ pub struct ExampleBlock<'s> {
     pub source: &'s str,
     pub switches: Option<&'s str>,
     pub number_lines: Option<SwitchNumberLines>,
+    pub preserve_indent: Option<CharOffsetInLine>,
     pub retain_labels: RetainLabels,
     pub use_labels: bool,
     pub label_format: Option<&'s str>,
@@ -71,6 +72,7 @@ pub struct SrcBlock<'s> {
     pub switches: Option<&'s str>,
     pub parameters: Option<&'s str>,
     pub number_lines: Option<SwitchNumberLines>,
+    pub preserve_indent: Option<CharOffsetInLine>,
     pub retain_labels: RetainLabels,
     pub use_labels: bool,
     pub label_format: Option<&'s str>,