Allow tab for indentation

By adding one field to the IndentConfig.t records, it is possible to customize the indentation character.



Note that using only the first commit (instead of both), makes it possible to 
use any arbitrary string (might be useful to pretty-print indentation levels 
as in

````
let look ma =
!  these
!  !  are
!  !  indentation
!  !  level;
!  (admittedly not that useful)
````