Issue230: Insert % before operators at the beginning of a cell

As described in #230

This feature is inspired by the use of four-function and graphing calculators especially. When you start a new calculation by typing an operator, it assumes you want to work with the `Ans` variable (the output of the previous operation).

The operators included in this functionality are:

```
+
*
/
^
=
, (e.g. for setting variables in the previous output expression)
```

Operators omitted are:

```
- (ambiguity: could be subtraction or negative)
~ (non standard)
. (ambiguity: dot product or decimal point)
```

These are at least the operators where I've felt the lack of this feature. There may be other operators that users are interested in.

The use case of subtracting a number from the previous answer (-) can be worked around by typing + followed by - (adding a negative number).

---

I added a configuration option to turn this feature off at the user's discretion. AFAICT the configuration option works as intended. I modeled the usage of the configuration option after the EditorCell::m_matchParens option.

---

Edit: updated the list of operators and omitted operators in description.