Add FieldLoader and linter support for BooleanExpression.

OpenRA.Game/Support/BooleanExpression.cs

@@ -18,6 +18,7 @@ namespace OpenRA.Support
 {
 	public class BooleanExpression
 	{
+		public readonly string Expression;
 		readonly HashSet<string> variables = new HashSet<string>();
 		public IEnumerable<string> Variables { get { return variables; } }
 
@@ -68,6 +69,7 @@ namespace OpenRA.Support
 
 		public BooleanExpression(string expression)
 		{
+			Expression = expression;
 			var openParens = 0;
 			var closeParens = 0;
 			var tokens = new List<Token>();