MRU_Toggle cmd added

[TamaMcGlinn]

Saw this pull request on a weird not-quite fork called vim-scripts/mru. Rebased, tested and works.

[yegappan]

Thanks for the patch. Can you also update the help text with
a description of this new command?

[TamaMcGlinn]

This was my first time actually documenting a vim plugin. I hope this was all that you meant.

[TamaMcGlinn]

I just read this:

By default, the MRU window is opened as the bottom-most window. If you are
using Vim version 8.0 and above, then you can use command modifiers like
|:topleft| or |:botright| or |:vertical| with the :MRU command to control
where the window is opened. Example: >

    :topleft MRU			" horizontally split topmost window
    :botright MRU			" horizontally split bottommost window
    :vertical topleft MRU		" vertically split far-left window
    :vertical botright MRU		" vertically split far-right window

and realised my mistake. I guess I need to put MRUToggle into the same if and pass the other args.

[TamaMcGlinn]

Should be good to merge now.