Move parsers for specific objects/elements to their own files.

The parsers are the most complicated part, so I want them in their own files. I am uncertain if I want to move their corresponding structs or just the parsers.

src/parser/mod.rs

@@ -6,8 +6,10 @@ mod greater_element;
 mod lesser_element;
 mod list;
 mod object;
+mod paragraph;
 mod parser_context;
 mod parser_with_context;
+mod plain_text;
 mod source;
 mod util;
 pub use document::document;