Improvements

I've added a few improvements to wintabs and also corrected a few bugs: 

# New options: `wipeout_buffers_oncloce`

If true, when closing a buffer, this will also be wiped out. The default is 0,
so the default behavior is not changes. 

# New function: `wintabs#previous`

This is changing to the last accessed tab. So, this way, you can easilly
change between the last 2 edited tabs. 

# Numbers in buffer names. 

This way we can very easilly call `WintabsGo`.

# Bug fix: Saving the tabslist to session

Since, according to the VIM documentation, only string and number variables
are saved using the `globals` in the `sessionoptions`, the tabs were not
restored after a session restore. I've fixed that by adding a new variable,
`g:Str_wintabs_session`, which holds the `string` serialization of
`g:Wintabs_session`. When the session is restores, the `g:Wintabs_session`
variable is restores from the string variable.

# Bug fix: Restoring the session might on empty windows

Sometimes, an empty window gets saved in the `g:Wintabs_session` variable.
When we encounter such a variable on restore, we just ignore it. If it's in
the second window, will break the session restore, since the window is not
restored, but the `w:wintabs_buflist` variable is cleared.