Add paragraph alignment setting (justify/left/center/right) (#191)

## Summary

* **What is the goal of this PR?** 

Add a new user setting for paragraph alignment, instead of hard-coding
full justification.

* **What changes are included?**

One new line in the settings screen, with 4 options
(justify/left/center/right). Default is justified since that's what it
was already. I personally only wanted to disable justification and use
"left", but I included the other options for completeness since they
were already supported.

## Additional Context

Tested on my X4 and looks as expected for each alignment.

Co-authored-by: Maeve Andrews <maeve@git.mail.maeveandrews.com>

lib/Epub/Epub/parsers/ChapterHtmlSlimParser.cpp

@@ -97,7 +97,7 @@ void XMLCALL ChapterHtmlSlimParser::startElement(void* userData, const XML_Char*
     if (strcmp(name, "br") == 0) {
       self->startNewTextBlock(self->currentTextBlock->getStyle());
     } else {
-      self->startNewTextBlock(TextBlock::JUSTIFIED);
+      self->startNewTextBlock((TextBlock::Style)self->paragraphAlignment);
     }
   } else if (matches(name, BOLD_TAGS, NUM_BOLD_TAGS)) {
     self->boldUntilDepth = std::min(self->boldUntilDepth, self->depth);
@@ -221,7 +221,7 @@ void XMLCALL ChapterHtmlSlimParser::endElement(void* userData, const XML_Char* n
 }
 
 bool ChapterHtmlSlimParser::parseAndBuildPages() {
-  startNewTextBlock(TextBlock::JUSTIFIED);
+  startNewTextBlock((TextBlock::Style)this->paragraphAlignment);
 
   const XML_Parser parser = XML_ParserCreate(nullptr);
   int done;