Counter expressions (#3534)

* Add peglib

* - Add expression engine
- Take an expression when setting a counter

* Shift + Click = Middleclick for counters

* minor cleanup for clangify

Signed-off-by: Zach Halpern <ZaHalpern+github@gmail.com>

* Added tip entry

tests/expression_test.cpp

@@ -0,0 +1,30 @@
+#include "gtest/gtest.h"
+#include "../common/expression.h"
+
+#include <cmath>
+
+#define TEST_EXPR(name,a,b) TEST(name, Works) {  \
+    Expression exp(8); \
+	ASSERT_EQ(exp.parse(a), b) << a; \
+}
+
+namespace
+{
+
+	TEST_EXPR(Number, "1", 1)
+	TEST_EXPR(Multiply, "2*2", 4)
+	TEST_EXPR(Whitespace, "3 * 3", 9)
+	TEST_EXPR(Powers, "2^8", 256)
+	TEST_EXPR(OrderOfOperations, "2+2*2", 6)
+	TEST_EXPR(Fn, "2*cos(1)", 2*cos(1))
+	TEST_EXPR(Variable, "x / 2", 4)
+	TEST_EXPR(Negative, "-2 * 2", -4)
+	TEST_EXPR(UnknownFnReturnsZero, "blah(22)", 0)
+
+} // namespace
+
+int main(int argc, char **argv)
+{
+    ::testing::InitGoogleTest(&argc, argv);
+    return RUN_ALL_TESTS();
+}