add GET with byte range about cpp-httplib

[yhirose]

@ReneKeilwerth, thanks for the good suggestion. I put labels 'enhancement' and 'HTTP/1.1'.

Only thing with the interface is that it doesn't handle the multi-part ranges like 'Range: bytes=0-50, 100-150, ..., ..., ...', and it doesn't sound a good idea to add more parameters on 'get' method.

https://developer.mozilla.org/en-US/docs/Web/HTTP/Range_requests#Multipart_ranges
https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Range#Syntax

I'll ponder over how to implement the 'get' method interface elegantly to support such a situation.

Thanks for your contribution!

[yhirose]

@ReneKeilwerth, the HTTP range has been supported.

auto s = httplib::make_range_header(1, 10, 100, 200, 300);
// 's.first' should be 'Range'
// 's.second' should be 'bytes=1-10, 100-200, 300-'

https://github.com/yhirose/cpp-httplib#range-http11