Update README
Add custom key modifier for bindings

Now possible to use own key modifier via 'g:move_key_modifier'. For example,

    let g:move_key_modifier = 'C'

create a following key bindings:

    <C-j> to move line/selection down
    <C-k> to move line/selection up
Add documentation for key modifier
Merge pull request #2 from vitalk/master

Allow to set custom key modifier to key bindings