Add option to offset first page in double page reader (#418)

* added toggle for doublespreads

* changing pages works as intended

* undid package.json changes

* deleted packege lock json

* curPage changes when changing invertDoublePage

* changed yarn.lock

* it switches better now

* fixed formatting

* changed it to offset first page

* fixed flipping when switching

* offset first page hides when double page is not enabled

* removed uncreachable line

src/i18n/locale/en.json

@@ -362,7 +362,8 @@
         "show_page_number": "Show page number",
         "skip_dup_chapters": "Skip duplicate chapters",
         "static_navigation": "Static navigation",
-        "fit_page_to_window": "Fit page to window"
+        "fit_page_to_window": "Fit page to window",
+        "offset_first_page": "Offset first page"
       },
       "reader_type": {
         "label": {
@@ -470,4 +471,4 @@
     },
     "title": "Updates"
   }
-}
+}