Add `MM_COL_NUMBER` to bundle environment.

Feature request please, to add environment variable MM_COL_NUMBER to the environment when invoking a bundle command. Analogous to MM_LINE_NUMBER, this new environment variable would indicate the column that the cursor is it.

My use case is that I have a bundle that inserts text templates into the composer window. It would be nice to be able to insert where the cursor is, rather than at the beginning of the line. Hopefully it would be useful for other bundles too.

Thanks for considering!

Cheers,
Steve.