[#257] Adopt "General/foo" instead of "general/foo".

It may be the Qt5 bug that stores "general" section as "[%General]",not
"[%general]" in NativeFormat on Linux. It will not bother windows user(s)
anyhow, since  the Windows registry use case-insensitive keys.
This should fix #257 and #249.

Author: Walter.Liu

app/options.cpp

@@ -20,8 +20,8 @@
 #include <QWebSettings>
 
 
-static const char* MARKDOWN_CONVERTER = "general/converter";
-static const char* LAST_USED_STYLE = "general/lastusedstyle";
+static const char* MARKDOWN_CONVERTER = "General/converter";
+static const char* LAST_USED_STYLE = "General/lastusedstyle";
 static const char* STYLE_DEFAULT = "actionDefault";
 static const char* FONT_FAMILY_DEFAULT = "Monospace";
 static const char* FONT_FAMILY = "editor/font/family";