Add support for binary compression of widget data

[andrjohns]

When creating files with multiple widgets and large input data, the resulting HTML filesize is dominated by the embedded JSON data.

This PR introduces the option (disabled by default): htmlwidget.compress_payload, to store widget data as a compressed binary string. Decompression is handled by the fflate library - only the unzlibSync and strFromU8 functions are bundled, resulting in only 5kb (minified) added to the output.

In our local testing this reduced the output size for one of our HTML files from 55mb -> 10mb, with little lag in the initial rendering time.

Let me know if I've missed anything, or if there's a better way to do this, thanks!