Reconciled parsing of field names with Substitution Field Spec. (#82)

According to the spec., valid syntax for merge field names is determined
by the merge source with a couple of minor exceptions.  Current text backends
allow any printable characters, so the SubstitutionField parser now accepts
any printable characters.

model/SubstitutionField.cpp

@@ -145,7 +145,7 @@ namespace glabels
 		{
 			bool success = false;
 		
-			while ( s.size() && (s[0].isDigit() || s[0].isLetter() || s[0] == '_' || s[0] == '-') )
+			while ( s.size() && (s[0].isPrint() && s[0] != ':' && s[0] != '}') )
 			{
 				field.mFieldName.append( s[0] );
 				s = s.mid(1);