Allow modification of menu keybindings

[Leandros]

My keyboard layout has different keys on 'j' & 'k'. This allows changing the defaults.

[PhilRunninger]

Two more things:

1. adjust the j/k text in https://github.com/scrooloose/nerdtree/blob/master/lib/nerdtree/menu_controller.vim#L52
2. Document the new variables in https://github.com/scrooloose/nerdtree/blob/master/doc/NERDTree.txt.

[Leandros]

Reasonable. Will add the changes later today.

[Leandros]

Incorporated your suggestions. Although, I didn't update the j/k text, since by default you'd still use j and k for navigation. The variables are documented in NERDTree.txt.

[PhilRunninger]

That text needs to change. The default is j/k, but if you override them, that needs to be reflected in the text.

    echo "NERDTree Menu. Use " . g:NERDTreeMenuDown . "/" . g:NERDTreeMenuUp . "/enter and the shortcuts indicated"

[Leandros]

Ahh, I didn't think of that. Makes sense.

[Leandros]

Fixed.

[PhilRunninger]

Excellent! Thanks for the contribution, @Leandros.

[Leandros]

Thank you for NERDTree!