Add metadata as columns between row-tree and row-panel

[tseemann]

In the latest version you support a tree lined up with each pan row.

It would be very useful to be able to add the metadata (currenly pan.strains I think) as columns in between the new tree and the pan genome. Each column of metadata should be able to be switched on or off.

                       Year  Country  MLST                            
    +--* ... Sample1   1973    AU     22    xxxxxxxxxxxxxxx
    |
 ---+---* .. Sample2   1984    USA    328   xxxxx xxxx xx
    |
    +-* .... Sample3   1969    UK     3     xxxxxxx  x xx xxxx xxx