Center padding option for Surface Duos/other foldables (#634)

* Add center margin option for horizontal dual page view

* Update readme to make the point of the fork obvious

* Center margins now added to large single images that would display in place of two smaller pages

* Cleanup; reworked preference into an int selector to specify which margins to add

* Suggested fixes and and readme reversion which somehow got skipped last commit

* Missed the build block in readme. Oops.

README.md

@@ -85,8 +85,8 @@ Please make sure to read the full guidelines. Your issue may be closed without w
 <details><summary>Bugs</summary>
 
 * Include version (More → About → Version)
- * If not latest, try updating, it may have already been solved
- * Preview version is equal to the number of commits as seen in the main page
+* If not latest, try updating, it may have already been solved
+* Preview version is equal to the number of commits as seen in the main page
 * Include steps to reproduce (if not obvious from description)
 * Include screenshot (if needed)
 * If it could be device-dependent, try reproducing on another device (if possible)
@@ -119,4 +119,4 @@ See [CODE_OF_CONDUCT.md](./CODE_OF_CONDUCT.md).
 ## FAQ
 
 [See our website.](https://tachiyomi.org/)
-You can also reach out to us on [Discord](https://discord.gg/tachiyomi).
+You can also reach out to us on [Discord](https://discord.gg/tachiyomi).