Fixed #38

Added a mapping `<Plug>(table-mode-sort)` which is mapped by default to
`<Leader>ts` that sorts the current table by the column in which the
cursor is currently.

Also added `TableSort` command that gives you a little more control over
the sorting, you can pass in additional :sort flags like u, i, n etc and
also use it with a `!` to sort in reverse order.