First attempt at text markup.

src/parser/lesser_element.rs

@@ -1,6 +1,6 @@
 use super::object::Object;
-use super::object::TextMarkup;
 use super::source::Source;
+use super::PlainText;
 
 #[derive(Debug)]
 pub struct Paragraph<'s> {
@@ -97,7 +97,7 @@ pub struct LatexEnvironment<'s> {
 impl<'s> Paragraph<'s> {
     pub fn of_text(input: &'s str) -> Self {
         let mut objects = Vec::with_capacity(1);
-        objects.push(Object::TextMarkup(TextMarkup { source: input }));
+        objects.push(Object::PlainText(PlainText { source: input }));
         Paragraph {
             source: input,
             children: objects,