add shortcuts to change how tabs are handled

Introduces mappings to quickly change how tabs are handled.  I find these mappings helpful when dealing with code that uses spaces rather than tabs and thought others might find it useful as well.

`<leader>ts2`, `<leader>ts4` (use spaces, not tabs with size 2 or 4 respectively)
`<leader>tt2`, `<leader>tt4` (use tabs, not spaces with size 2 or 4 respectively)