add settings for reader screen margin (#223)

## Summary

* **What is the goal of this PR?** 
* This PR adds a setting to control the top left and right margins of
the reader screen in 4 sizes (5, 10, 20, 40 pt?) and defaults to `SMALL`
which is equivalent to the fixed margin of 5 that was already in use
before.
* **What changes are included?**

## Additional Context

* Add any other information that might be helpful for the reviewer
(e.g., performance implications, potential risks, specific areas to
focus on).

---------

Co-authored-by: Dave Allie <dave@daveallie.com>

src/CrossPointSettings.cpp

@@ -12,7 +12,7 @@ CrossPointSettings CrossPointSettings::instance;
 namespace {
 constexpr uint8_t SETTINGS_FILE_VERSION = 1;
 // Increment this when adding new persisted settings fields
-constexpr uint8_t SETTINGS_COUNT = 13;
+constexpr uint8_t SETTINGS_COUNT = 14;
 constexpr char SETTINGS_FILE[] = "/.crosspoint/settings.bin";
 }  // namespace
 
@@ -40,6 +40,8 @@ bool CrossPointSettings::saveToFile() const {
   serialization::writePod(outputFile, paragraphAlignment);
   serialization::writePod(outputFile, sleepTimeout);
   serialization::writePod(outputFile, refreshFrequency);
+  serialization::writePod(outputFile, screenMargin);
+
   outputFile.close();
 
   Serial.printf("[%lu] [CPS] Settings saved to file\n", millis());
@@ -92,6 +94,9 @@ bool CrossPointSettings::loadFromFile() {
     if (++settingsRead >= fileSettingsCount) break;
     serialization::readPod(inputFile, refreshFrequency);
     if (++settingsRead >= fileSettingsCount) break;
+    serialization::readPod(inputFile, screenMargin);
+    if (++settingsRead >= fileSettingsCount) break;
+
   } while (false);
 
   inputFile.close();
@@ -207,4 +212,4 @@ int CrossPointSettings::getReaderFontId() const {
           return OPENDYSLEXIC_14_FONT_ID;
       }
   }
-}
+}