feat(scopes): Implement granular OAuth scopes

BREAKING CHANGE: Applications that use the `Scopes` data structure will
have minor changes to make

Closes #44

examples/register.rs

@@ -31,7 +31,7 @@ pub fn register() -> Result<Mastodon, Box<Error>> {
     let website = read_line("Please enter your mastodon instance url:")?;
     let registration = Registration::new(website.trim())
         .client_name("elefren-examples")
-        .scopes(Scopes::All)
+        .scopes(Scopes::all())
         .website("https://github.com/pwoolcoc/elefren")
         .build()?;
     let url = registration.authorize_url()?;