Do not trip on new warn before read feature

Upstream privatebin introduced this new feature  PrivateBin/PrivateBin#1237

If the fragment contains a prefix character `-` , then the ui is supposed to warn before actyally getting paste, as the paste may be burn on read.

This character can trip our reading of the bs58key. So, strip the prefix if found.

src/main.rs

@@ -28,10 +28,13 @@ fn create_dataurl(path: &std::ffi::OsStr, data: String) -> String {
 fn handle_get(opts: &Opts) -> PbResult<()> {
     let url = opts.get_url();
     let paste_id = opts.get_url().query().unwrap();
-    let key = opts
+    let fragment = opts
         .get_url()
         .fragment()
         .ok_or(PasteError::MissingDecryptionKey)?;
+    // '-' character may be found at start of fragment. This should be stripped.
+    // It is used to activate "warn before read" feature for burn on read pastes.
+    let key = fragment.strip_prefix('-').unwrap_or(fragment);
 
     let api = API::new(url.clone(), opts.clone());
     let paste = api.get_paste(paste_id)?;