Allow if/else if.../else statements

Fix #37.

Author: polybuildr

src/grammar.rustpeg

@@ -50,6 +50,15 @@ if_statement -> Stmt
             }
         )
     }
+    / IF __ e:expr_node __ lpos1:#position b1:block rpos1:#position __ ELSE __ lpos2:#position elseif:if_statement rpos2:#position __ {
+        Stmt::IfThen(
+            IfThenStmt {
+                cond: e,
+                then_block: Box::new(StmtNode { pos: (lpos1, rpos1), data: b1 }),
+                maybe_else_block: Some(Box::new(StmtNode { pos: (lpos2, rpos2), data: elseif })),
+            }
+        )
+    }
     / IF e:expr_node lpos:#position b:block rpos:#position {
         Stmt::IfThen(
             IfThenStmt {

tests/run-pass/if-else-if.bl

@@ -0,0 +1,19 @@
+var x = 0;
+if false {
+    x = 1;
+} else if false {
+    x = 2;
+} else {
+    x = 3;
+}
+assert_eq(x, 3);
+
+var y = 0;
+if false {
+    y = 1;
+} else if true {
+    y = 2;
+} else {
+    y = 3;
+}
+assert_eq(y, 2);