Add toggle command

I added the following things to toggle the BufExplorer list on and off.
- the command `:ToggleBufExplorer`
- the map `<Leader>bt`, and 
- the function `ToggleBufExplorer()`

The function works by simply calling the `BufExplorer()` or `s:Close()` function, depending on the current state of BufExplorer. Now one can easily see which buffers are open (with a mapped function key, ideally), and then easily close the buffer list with the same key. 

I believe this pull request will satisfy the request in Issue #3.