add OldStyleStringToDifficulty method

because SM recognizes them in .sm files.

Docs/Luadoc/Lua.xml

@@ -115,6 +115,7 @@
 		<Function name='MonthToString'/>
 		<Function name='MultiPlayerToLocalizedString'/>
 		<Function name='MultiPlayerToString'/>
+		<Function name='OldStyleStringToDifficulty'/>
 		<Function name='OptionRowProTiming'/>
 		<Function name='OptionRowScreenFilter'/>
 		<Function name='OptionsRandomJukebox'/>

Docs/Luadoc/LuaDocumentation.xml

@@ -372,6 +372,9 @@ save yourself some time, copy this for undocumented things:
 		<code>next</code> returns the next index of the table and its associated value."
 		See the Lua manual for more details.
 	</Function>
+	<Function name='OldStyleStringToDifficulty' return='Difficulty' arguments='string dc'>
+		Converts a string such as 'oni' or 'expert' or 'trick' to the appropriate difficulty.
+	</Function>
 	<Function name='OptionRowProTiming' theme='default' return='LuaOptionRow' arguments=''>
 		[03 ThemePrefs.lua] Returns a Lua option row for ProTiming.
 	</Function>

src/Difficulty.cpp

@@ -72,6 +72,8 @@ Difficulty OldStyleStringToDifficulty( const RString& sDC )
 	else							return Difficulty_Invalid;
 }
 
+LuaFunction( OldStyleStringToDifficulty, OldStyleStringToDifficulty(SArg(1)) );
+
 static ThemeMetric<RString> NAMES("CustomDifficulty","Names");
 
 RString GetCustomDifficulty( StepsType st, Difficulty dc, CourseType ct )