Added option to customize indent keybindings.

Changes how indenting list items works. The user may now control which
keys perform the indenting (beyond choosing Tab/Alt-Right). There is a new
flag, g:notes_indents that controls whether the keybindings get set or
not. If the user has not set g:notes_indents and g:notes_tab_indents is
set, then g:notes_indents_right is set to Tab, g:notes_indents_left is set
to S-Tab and g:notes_indents is turned on (g:notes_alt_indents has a
similar behavior). If the user has set g:notes_indents, then she must also
provide g:notes_indents_right and g:notes_indents_left is order to specify
which keys to use. Currently, it's not possible to have multiple
keybindings to indent the list. The order of precedence is custom, Tab,
then Alt-Right.

Example .vimrc config for custom keybindings:

let g:notes_indents = 1
let g:notes_indents_right = ">>"
let g:notes_indents_left = "<<"

This will disable the Tab/Alt-Right keybindings and make >>/<< the
keybindings to indent the list items.