Update the documentation

* The feed item limit defaults to the default page size (50) if not
  provided
* Move caching from response to URL fetch results; add helper functions
* Add a helper function to set the paging query of an URL
* Modify paging so we don't retrieve more than the feed item limit

src/mixcloud.rs

@@ -112,7 +112,7 @@ const DEFAULT_FILE_TYPE: &str = "audio/mpeg";
 const DEFAULT_PAGE_SIZE: usize = 50;
 
 /// Returns the default file type used by Mixcloud.
-pub(crate) const fn default_file_type() -> &'static str {
+pub(crate) fn default_file_type() -> &'static str {
     DEFAULT_FILE_TYPE
 }