cf. https://docs.rs/httparse/1.3.4/httparse/constant.EMPTY_HEADER.html#example

Rather then looking at a slice of bytes is easier (for most humans) to
read a string. Since most, but not all, header values are strings
attempt to create a string from the header value and print that, falling
back to printing it a slice of bytes if the value is not a valid UTF-8
string.

The SIMD intrinsics *_cmpgt_epi8 are for signed chars.
This change correctly performs the unsigned comparison.

This provides a configuration option to enable obs-folded headers. It defaults to off. If enabled, the header value will include all the newlines, and thus will need to be replaced with a space afterwards.

Closes #37
Closes #68