Subhas | Added an auto-incrementing counter variable

Hi, I've added a counter `i` which automatically increments for each selection when running the Calculate command. For example:

Selecting the below text on the left, and running Calculate would produce:
```
i                   =>          0
i                   =>          1
i                   =>          2
```
```
i+10                =>          10
i+10                =>          11
i+10                =>          12
```
```
(i+10)*100          =>          1000
(i+10)*100          =>          1100
(i+10)*100          =>          1200
```
etc.

This is very useful for me since I can now apply linearly increasing mathematical calculations. It doesn't affect any existing functionality, the existing Calculate plugin will run as it is, only people wanting a counter variable can simply use `i`.