Log Message:
(math/R-gtable) Updated 0.3.4 to 0.3.6

# gtable 0.3.6

* Added `as.gtable()` S3 method (#97).
* Add `clip` argument to `gtable_col()` and `gtable_row()` (#56)
* Indexing a gtable with `NA` will now insert a zero-dimension row/column at the
  position of the `NA`-index (#13)

# gtable 0.3.5

* Fixed partial matching issue when constructing viewport in `gtable()` (#94)
* General upkeep