new global options for table alignment -- fix #38

Quick demo:

```rout
> f <- function(df) {
+         if (!is.table(df) || length(dim(df)) < 2)
+             ifelse(sapply(df, is.numeric), 'right', 'left')
+         else
+             ifelse(apply(df, 2, is.numeric), 'right', 'left')
+ }
> panderOptions('table.split.table', Inf)
> panderOptions('table.alignment.default',  f)
> pander(head(iris))

-------------------------------------------------------------------
  Sepal.Length   Sepal.Width   Petal.Length   Petal.Width Species  
-------------- ------------- -------------- ------------- ---------
           5.1           3.5            1.4           0.2 setosa   

           4.9             3            1.4           0.2 setosa   

           4.7           3.2            1.3           0.2 setosa   

           4.6           3.1            1.5           0.2 setosa   

             5           3.6            1.4           0.2 setosa   

           5.4           3.9            1.7           0.4 setosa   
-------------------------------------------------------------------
```