Implementation of relative split.cells

I have added support of supplying split.cells as a vector with percentage values.
It is relative to split.tables param, meaning that in that case split.tables becomes a max.width for a table.
Also, my implementation imposes a requirement that value is supplied for every column and rownames.

***Examples***
```
y <- c("aa aa aa", "aaa aaa", "a a a a a", "aaaaa", "bbbb bbbb bbbb", "bb bbb bbbb")
y <- matrix(y, ncol = 3, nrow = 2)
rownames(y) <- c("rowname one", "rowname two")
colnames(y) <- c("colname one", "colname two", "colname three")
```
```
d> pander(y, split.cells = c("10%","70%","10%","10%"))

---------------------------------------------
  &nbsp;     colname one   colname   colname 
                             two      three  
----------- ------------- --------- ---------
 **rowname    aa aa aa     a a a a    bbbb   
   one**                      a       bbbb   
                                      bbbb   

 **rowname     aaa aaa      aaaaa    bb bbb  
   two**                              bbbb   
---------------------------------------------
```
```
d> pander(y, split.cells = c("11%","67%","5%","17%"))

---------------------------------------------------
  &nbsp;     colname one   colname   colname three 
                             two                   
----------- ------------- --------- ---------------
 **rowname    aa aa aa       a a       bbbb bbbb   
   one**                     a a         bbbb      
                              a                    

 **rowname     aaa aaa      aaaaa     bb bbb bbbb  
   two**                                           
---------------------------------------------------
```
```
d> pander(y, split.cells = c("11%","67%","22%"))

-------------------------------------------------------------
     &nbsp;        colname one   colname two   colname three 
----------------- ------------- ------------- ---------------
 **rowname one**    aa aa aa      a a a a a   bbbb bbbb bbbb 

 **rowname two**     aaa aaa        aaaaa       bb bbb bbbb  
-------------------------------------------------------------

Warning message:
In pandoc.table.return(...) :
  Using relative split.cells require a value for every column and rownames. Reverting to default
```
```
d> pander(y, split.cells = c("11%","67%","11%", "50%"))

-------------------------------------------------------------
     &nbsp;        colname one   colname two   colname three 
----------------- ------------- ------------- ---------------
 **rowname one**    aa aa aa      a a a a a   bbbb bbbb bbbb 

 **rowname two**     aaa aaa        aaaaa       bb bbb bbbb  
-------------------------------------------------------------

Warning message:
In pandoc.table.return(...) :
  Supplied relative values don't add up to 100%. Reverting to default
```