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

Made a range of internal changes to increase performance of gtable construction, these include:
- Use more performant data.frame constructor .
- Treat layout data.frame as list when indexing and modifying it.
- Use length of widths and heights fields instead of ncol() and nrow() internally.
- Substitute stopifnot(...) with if(!...) stop().
Better documentation, including a new README, a vignette on performance profiling and a pkgdown site.
New logo
It is now an error to index into a gtable with non-increasing indices.
Dimnames are now inherited from the grobs data in gtable_col(), gtable_row(), and gtable_matrix()
gtable_trim now works with empty gtables
gtable_filter now has an invert argument to remove grops matching a name.

Switch from preDrawDetails() and postDrawDetails() methods to makeContent() and makeContext() methods (@pmur002, #50). This is a better approach facilitiated by changes in grid. Learn more at https://journal.r-project.org/archive/2013-2/murrell.pdf.
Added a NEWS.md file to track changes to the package.
Partial argument matches have been fixed.
Import grid instead of depending on it.

print.gtable now prints the z order of the grobs, and it no longer sort the names by z order. Previously, the layout names were sorted by z order, but the grobs weren't. This resulted in a mismatch between the names and the grobs. It's better to not sort by z by default, since that doesn't match how indexing works. The zsort option allows the output to be sorted by z.