new feature: replace expression with its evaluated result

[derhuerst]

Suppose i have a list of expressions:

- foo 1 + 2.3
- bar 2 + 3.4
- baz 3 + 4.5

It would be very handy to be able to select all of these expressions and, for each of the multiple cursors, replace the selection with the result it evaluates to!

The result would be this:

- foo 3.3
- bar 5.4
- baz 7.5