Table.expand

I have implemented table.expand in Rcpp. I did a lot of comparative testing with original table.expand, everything seems to be correct and work fine.

As for benchmarks, it is kind of controversial. General speed up of pandoc.table.return is between 1.5 - 5x. For example
1) Small data.frame
```
> b.old.simple
Unit: milliseconds
                                                         expr      min       lq   median       uq      max neval
 pander(data.frame(a = "foo\\nbar"), keep.line.breaks = TRUE) 5.501682 6.224382 7.290573 8.382554 12.23485   100
> b.new.simple
Unit: milliseconds
                                                         expr     min       lq   median       uq      max neval
 pander(data.frame(a = "foo\\nbar"), keep.line.breaks = TRUE) 2.30613 2.587211 2.924714 3.806796 5.605237   100
```
2) mtcars
```
> b.old.mtcars
Unit: milliseconds
                             expr      min       lq   median       uq      max neval
 pander(mtcars, style = "simple") 192.7679 201.4737 206.0359 210.8648 282.0304   100
> b.new.mtcars
Unit: milliseconds
                             expr     min       lq   median       uq      max neval
 pander(mtcars, style = "simple") 66.4057 73.86317 76.89688 83.96956 119.5512   100
```
3) volcano
```
> b.old.volcano
Unit: seconds
            expr      min       lq   median       uq      max neval
 pander(volcano) 4.408776 4.415709 4.479123 4.513334 4.536083    10
> b.new.volcano
Unit: seconds
            expr      min       lq   median       uq      max neval
 pander(volcano) 3.502687 3.545587 3.583752 3.634691 3.850749    10
```

So for really big datasets (volcano is one of the biggest in `datasets` package) speed it is getting smaller, but for other ones it is better.
One reason I am not fully satisfied with results of `pandoc.table` speedup is that `table.expand` speedup is much better (10-25x).

```
> b1
Unit: microseconds
                                                              expr     min      lq   median      uq      max neval
           table.expand(argv[, 1], argv[, 2], argv[, 3], sep.cols) 711.426 743.061 810.3655 863.430 2219.897   100
 tableExpand_cpp(argv[, 1], argv[, 2], argv[, 3], sep.cols, style)  57.685  64.676  84.1530 100.071  476.688   100
> b2
Unit: microseconds
                                                              expr      min        lq    median       uq      max neval
           table.expand(argv[, 1], argv[, 2], argv[, 3], sep.cols) 1929.499 1989.7145 2066.2760 2599.152 4317.919   100
 tableExpand_cpp(argv[, 1], argv[, 2], argv[, 3], sep.cols, style)   58.566   65.9905   85.2165   96.538  167.777   100
> b3
Unit: microseconds
                                                              expr      min       lq    median       uq      max neval
           table.expand(argv[, 1], argv[, 2], argv[, 3], sep.cols) 1366.966 1405.690 1455.6460 1502.291 3627.009   100
 tableExpand_cpp(argv[, 1], argv[, 2], argv[, 3], sep.cols, style)   55.687   67.638   87.1345   96.651  190.005   100
> b4
Unit: microseconds
                                                              expr      min        lq    median       uq      max neval
           table.expand(argv[, 1], argv[, 2], argv[, 3], sep.cols) 1326.334 1365.1385 1384.4160 1412.322 3128.808   100
 tableExpand_cpp(argv[, 1], argv[, 2], argv[, 3], sep.cols, style)   54.686   65.6485   78.1735   91.113  134.672   100
> b5
Unit: microseconds
                                                              expr      min        lq   median       uq      max neval
           table.expand(argv[, 1], argv[, 2], argv[, 3], sep.cols) 1326.983 1378.6020 1395.999 1414.224 3168.186   100
 tableExpand_cpp(argv[, 1], argv[, 2], argv[, 3], sep.cols, style)   55.294   68.7475   87.765   92.075  164.647   100
```

So it seems to me that there is still good room for improvement and that for big tables there are other bottlenecks.
Also I have realized that there is no S3 method for `microbenchmark` in `pander`, so I am planning to add that also.