Make it easy to evaluate expressions with simple subscripts from JS

> KAS.parse("x_0 + x_a").expr.eval({x_0: 1, x_a: 2}) === 3
true

You can now do the above instead of `.eval({"x_(0)": 1, "x_(a)": 2})`

Test Plan:
New tests in test.html

Auditors: eater