[MLton] Added completion to esml-mlb-mode

[Vesa Karvonen]

I have added context sensitive completion for annotations, paths and path
variables to esml-mlb-mode. Completion is activated by pressing tab. (If
indentation doesn't change, completion is attempted.) Completion can also
be turned off (by a customization variable), but it is on by default.

I have tested the completion interactively on both GNU Emacs (21.4) and
XEmacs (21.4) (I primarily use XEmacs). Please report any bugs you find.

-Vesa Karvonen