Respect ini settings for file uploads

[clue]

Now that we've started offering a way to buffer incoming request bodies (#216 and #235) and parse incoming request bodies (#220 and #226), we may want to look into supporting PHP's default ini settings to control this behavior.

For example, these are the relevant default settings on my Ubuntu box:

file_uploads		On
max_file_uploads	20
max_input_nesting_level	64
max_input_vars		1000
upload_max_filesize	2M

post_max_size		128M
max_input_time 		-1 # hardcoded for CLI
memory_limit	 	256M # parallel requests?

See also http://php.net/manual/en/info.configuration.php and http://php.net/manual/en/ini.core.php.

[clue]

I'd like to make this actionable, so see #255 and #257 instead 👍